Add Usage Recommendations section

fragments/usage/README.md

@@ -0,0 +1,20 @@
+# Usage Recommendations Fragment
+
+This fragment describes conditions under which the model publishers believe the model will perform
+as expected. 
+
+While the [Model Training] section gives users a clear picture of the training conditions, the model
+publisher may also have evidence or intuition that the model will also perform well when used in a 
+different context. For instance, a model might have been trained on data from a particular
+geographic region and time period, but based on some combination of intuition, previous experience,
+and informal experimentation, the model publisher may be confident that it will perform similarly in
+a new geographic region. This section is meant to capture that information in a systematic way,
+while also providing publishers the opportunity to describe their recommendations in a more
+free-form style.
+
+## Directory Contents
+
+* [`usage-fragment.md`](./usage-fragment.md) - The detailed Usage Recommendations Fragment
+  specification.
+
+[Model Training]: ../training

fragments/usage/usage-fragment.md

@@ -0,0 +1,80 @@
+# Usage Recommendations Specification
+
+The Usage Recommendations section defines conditions under which the model publishers believe the
+model will perform as expected. It also provides a mechanism for publishers to explicitly call out
+conditions under which they believe the model is likely to not performa as expected. It is important
+to note that these are *recommendations* and that they **in no way provide any guarantee of model 
+performance under the given conditions**.
+
+If no Usage Recommendation section is provided, then users should assume that the model will only
+perform as expected under conditions that match those described in the [Model Training] section.
+This also applies to any fields that are not present in the Usage Recommendations section. For
+instance, if no `temporal` field is provided, users should assume the model will only perform as
+expected for data in the time range given in the [Model Training] section.
+
+
+## Usage Recommendations Fields
+
+The `recommendations` and `cautions` fields in this section are arrays of objects that describe
+conditions under which the model either should perform as expected or should *not* behave as
+expected, respectively.
+
+If the Usage Recommendations section is present, then at least one of these fields is required and
+must be a non-empty array.
+
+| Field Name        | Type                  | Description                                                                                    |
+| ----------------- | --------------------- | ---------------------------------------------------------------------------------------------- |
+| `recommendations` | \[[Usage Conditions]] | An array of objects describing conditions under which the model *should* perform as expected.  |
+| `cautions`        | \[[Usage Conditions]] | An array of objects describing conditions under which the model *may not* perform as expected. |
+
+## Usage Conditions
+
+This object is used to described conditions under which model usage is either recommended (if found
+in the `recommendations` list) or discouraged (if found in the `cautions` list).
+
+| Field Name  | Type                     | Description                                                                                |
+| ----------- | ------------------------ | ------------------------------------------------------------------------------------------ |
+| spatial     | [Spatial Extent Object]  | The spatial extents to which the recommendation or caution applies.                        |
+| temporal    | [Temporal Extent Object] | The temporal extents to which the recommendation or caution applies.                       |
+| description | string                   | A human-readable description of conditions to which the recommendation or caution applies. |
+
+### Spatial Extent Object
+
+| Element | Type         | Description                                                                                  |
+| ------- | ------------ | -------------------------------------------------------------------------------------------- |
+| bbox    | \[\[number]] | **REQUIRED.** Potential *spatial extents* within which the model should perform as expected. |
+
+The `bbox` element is an *array* whose elements are themselves arrays. Each inner array is a bounding 
+box that describes a spatial extent within which the model should perform as expected.
+
+The format of the inner bounding box arrays follows the specification outlined in the **bbox**
+section of the [STAC Spatial Extent Object] documentation. However, the `bbox` element defined in
+this spec differs from the `bbox` element in the STAC Spatial Extent Object in that there is no
+first element that always represents the overall spatial extent. Such an overall extent might give
+users the incorrect impression that the model should perform as expected anywhere within that
+spatial extent, when in fact there may be only specific areas in which this is true.
+
+### Temporal Extent Object
+
+| Element  | Type               | Description                                                                                   |
+| -------- | ------------------ | --------------------------------------------------------------------------------------------- |
+| interval | \[\[string\|null]] | **REQUIRED.** Potential *temporal extents* within which the model should perform as expected. |
+
+The `interval` element is an *array* whose elements are themselves arrays. Each inner array is an
+interval array containing 2 elements, each of which may be either `null` or a timestamp. Timestamps
+must follow the specifications outlined in the [STAC Temporal Extent Object] documentation.
+
+The format of the inner interval arrays follows the specification outlined in the [STAC Temporal
+Extent Object] documentation. However, the `interval` element defined in this spec differs from 
+the `interval` element in the STAC Temporal Extent Object in that there is no first element that 
+always represents the overall temporal extent. Such an overall extent might give users the 
+incorrect impression that the model should perform as expected anywhere within that temporal range, 
+when in fact there may be only specific ranges in which this is true.
+
+
+[Model Training]: ../training
+[Spatial Extent Object]: #spatial-extent-object
+[Temporal Extent Object]: #temporal-extent-object
+[Usage Conditions]: #usage-conditions
+[STAC Spatial Extent Object]: https://github.com/radiantearth/stac-spec/blob/v1.0.0-rc.4/collection-spec/collection-spec.md#spatial-extent-object
+[STAC Temporal Extent Object]: https://github.com/radiantearth/stac-spec/blob/v1.0.0-rc.4/collection-spec/collection-spec.md#temporal-extent-object

model-metadata/model-metadata.md

@@ -91,10 +91,6 @@ This object describes a single model output.
 | `shape`       | [int]   | **REQUIRED.** The shape of the parameter as an array of integers.      |
 | `description` | string  | A human-readable description of the parameter that indicates what type of content it contains. |
 
-### Usage Recommendations
-
-**COMING SOON**
-
 [License identifier]: https://spdx.org/licenses/
 [STAC Scientific Citation Extension]: https://github.com/radiantearth/stac-spec/tree/v1.0.0-rc.1/extensions/scientific
 [Runtime]: ../fragments/runtime
@@ -105,5 +101,5 @@ This object describes a single model output.
 [Citation]: #citation
 [Model Input]: #model-input
 [Model Output]: #model-output
-[Usage Recommendations]: #usage-recommendations
+[Usage Recommendations]: ../fragments/usage
 [Publication Object]: #publication-object