Block trade

A block trade aggregates the same order across many client accounts into a single larger order, then allocates the fills back to each account.

Blocking exists to treat clients fairly and to reduce cost. Every account in the block receives the same average price, so no client is advantaged by the order in which their account happened to be traded.

It requires an allocation policy written in advance, because deciding who gets which fill after the fact is precisely the conflict the practice is meant to remove.