The Allocation section mixes Server Allocation settings with the Maximum rows per server and Use a subset of the seed data settings to arrive at a powerful way to slice data.

The “Server allocation” drop-down controls how the data is divided among the servers in a multi-server Composition.

Separate slices of the seed data will be sent to each server

Choose this to divide the data evenly among all of the servers, and send a unique slice of the data to each server.