Agencies/big brands look at each digital channel and how much they could potentially spend reaching their target market and then assign budgets months in advanc

Agencies/big brands look at each digital channel and how much they could potentially spend reaching their target market and then assign budgets months in advance. So if the potential spend on Facebook is lower, it may get less budget in the plan because the agency knows they won’t be able to spend more budget there. My initial wording is weird.