The following overrides are available.
Override bidding strategies
A bid strategy can only be set at the campaign level. So for example, if a campaign has already been assigned a TARGET_ROAS strategy, setting TARGET_CPA fields in one of its ad groups will have no effect.
Similarly, setting bids manually will have no effect if you are using a campaign-level bid strategy that doesn't support them. For example, if your campaign uses TARGET_ROAS as a standard bidding strategy, and you set max CPC values for individual keywords in that campaign, the keyword-level bids will be ignored without errors. If you were to switch to manual bidding, the keyword-level bids would then take effect. Read more in the Help Center.
Override Target ROAS at the ad group level
The target_roas field of a TargetRoas bidding scheme, or a MaximizeConversionValue with its target_roas field set, can be overridden at the ad group level by setting the ad group's target_roas field. The other fields of the bidding scheme cannot be overridden.
To remove an override, unset target_roas of the ad group by sending a mutate request with an empty field value but still supplying the path in a field mask. Attempting to override the target_roas field of a TargetRoas scheme associated to a portfolio strategy will result in an error.
You can retrieve the effective target_roas value of an ad group with the effective_target_roas field and its source with the effective_target_roas_source field.
Override Target CPA at the ad group level
The target_cpa_micros field of a TargetCpa bidding scheme, or a MaximizeConversions with its target_cpa_micros field set, can be overridden at the ad group level by setting the ad group's target_cpa_micros field. The other fields of the bidding scheme cannot be overridden.
To remove an override, unset target_cpa_micros of the ad group by sending a mutate request with an empty field value but still supplying the path in a field mask.
You can retrieve the effective value of target_cpa_micros for an ad group from the effective_target_cpa_micros field and its source from the effective_target_cpa_source field.