The targeting setting determines if an ad group must serve ads only to users in the associated remarketing lists (Target and Bid option) or to all users (Bid only option).
It is not logically possible for an ad group to be associated with one remarketing list using the Target and Bid option and another using the Bid only option. Hence, the system enforces the same targeting setting across all the associations for a given ad group.
While the status and bid adjustments of associations can be edited in bulk by selecting associations and clicking Edit on the Audience tab, targeting setting cannot. Targeting setting can only be edited one ad group at a time from the Audience tab by clicking on Add association and then selecting the relevant ad group. The updated targeting setting is applied to all the associations of that ad group.