Why are funds reserved?

To prevent attacks and make sure the blockchain is storing data efficiently, some funds are reserved as soon as some data is stored on the blockchain.

Multisig transaction

When you create a multisig transaction, the data of the transaction is stored on-chain, as long as it hasn't been approved or canceled. On Polkadot, the reserved funds are ~20 DOTs. The funds will be returned when the transaction is approved (with enough signatories signing it) or canceled.

Pure proxy creation

When you create a pure proxy, the chain needs to store the association between the pure proxy, and the account. On Polkadot the reserved funds for this storage is ~20 DOTs. The funds will be returned when the proxy is "killed".

Sum up

If you create a multisig, with a proxy, you must have the funds needed to pay for the reserve of

• the transaction to create a pure proxy (returned as soon as it's approved)
• the pure proxy (only returned if the pure proxy is killed, which may be far in the future)

For this reason you will need to have ~40 DOTs, 20 of which will be returned in the short term, and 20 which will remained reserved for as long as you need the multisig/pure.