These two mechanisms look similar on a screen and are opposite in kind. Getting them the wrong way round either oversells the house or refuses business you could have taken.
A restriction — stop-sell, closed-to-arrival, closed-to-departure, minimum or maximum stay — is a statement about the property. It binds every channel including your own desk. A room type closed for maintenance is closed, and a clerk selling it because "the restriction was meant for the OTAs" is exactly the failure the flag exists to prevent.
An allotment is a contract with a named counterparty: this agent may sell ten of these. It caps that channel and must never cap the house. The one deliberate exception is a row written explicitly against the house, which is the property's own declared ceiling on its own pool.
A group block is a third thing again — it consumes inventory rather than capping a channel. Its own delegates can be booked against it without being refused by the hold it created, which is the difference between protecting a group and trapping one.