Traffic & geos
Audience pool
The reachable user-set within a given platform/geo/targeting set.
Audience pool is the set of reachable users within a given targeting layer. Audience pools deplete: as you spend, the high-converting users get acquired, the remainder converts at lower rates ('audience fatigue'). Operators expand pools (broaden targeting) or refresh creatives to re-acquire impression-share among already-saturated audiences. Pool size estimates: Meta Ads Manager 'audience size,' Google's 'estimated reach,' YouTube CTV 'available impressions.'
Example
Meta campaign at $200/day saturates a 1.8M-person audience pool over six weeks (CPA rises from $34 to $58); operator expands targeting (state→country) and refreshes creative to lift CPA back to $38.