Concepts
The x/hold module is designed for use by other modules that need to lock funds in place in an account.
Holds
"Holds" are an amount of funds in an account that must not be moved out of the account.
When a hold is placed on some funds, a record of them is created in the x/hold module.
When funds are released from a hold, those records are updated appropriately.
Funds with a hold on them remain in the owners account but cannot be spent, sent, delegated or otherwise removed from the account until they are released from hold.
A hold can only be placed on funds that would otherwise be spendable. E.g. you can place a hold on vested funds, but not unvested funds.
Managing Holds
The x/hold module does not have any Msg or Tx endpoints for managing holds.
Putting holds on funds and releasing holds are actions that are only available via keeper functions.
It is expected that other modules will use the keeper functions (e.g.AddHold and ReleaseHold) as needed.
Locked Coins
The x/hold module injects a GetLockedCoinsFn into the bank keeper in order to tell it which funds have a hold on them.
This allows the bank module and keeper functions to take holds into account when reporting bank account information.
Specifically, the bank keeper functions, LockedCoins, and SpendableCoins will reflect holds, as well as the SpendableBalances query.
The AllBalances query and similar keeper functions will still include the held funds though, since the funds actually are still in the account.