Media mix model
A media mix model is a statistical model of how each channel's spend drives sales, calibrated with lift tests.
A move between channels compares counts each platform makes its own way, so it needs a lift test or a media mix model. A lift test measures incrementality, and the media mix model is calibrated with lift tests. Without either, the brand manager judges the shift on the brand's own sales.
The best case for handing an agent the split across channels is that a media mix model already recommends one, and the agent would only apply it faster. But the model and the lift tests behind it lag the spend, and its split predates the brand's current margins, stock, and commitments. None of those sit in an ad account, so a person reviews and approves each shift.
Example
When the agent drafts a shift from social to retail media for a snack brand, the brand manager decides. The model's split predates commitments such as a CPG brand's retailer agreements, and the brand manager weighs those as they change.