Replaces the campaign assignments of a spend cap with the given campaign_ids. Campaigns missing from the list stop being assigned to the spend cap as of effective_from. Campaigns new to the list are assigned as of effective_from. Campaigns already on the list are left unchanged.

Changes take effect per calendar day in the platform's time zone. Any time you pass in effective_from is treated as 00:00 of that day, and if you leave it out the change takes effect from 00:00 today.

A campaign's assignment can change at most once per day, due to the daily granularity of spend attribution, so a change that takes effect today is rejected if any campaign it adds or removes:

• is currently assigned to a spend cap. As a result, removing a campaign, or moving it to another spend cap, can only take effect from tomorrow onward.
• had an assignment to any spend cap that ended earlier today.
• has an assignment to any spend cap that is scheduled to start later today.

If any campaign in the request is rejected, the whole request fails and no assignments are changed. Changes that take effect from tomorrow onward are not subject to this rule.