Expiration fields available within a campaign will override the redeemable's (associated with said campaign) expiration rules.
For example, below is an image of an Anniversary Campaign expiration fields. This means that the redeemable gifted for the user's anniversary will expire 6 days after it has been gifted.
The redeemable that is gifted for the Anniversary campaign has the below expiration settings. The redeemable is set to expire 14 days after it is gifted.
If the redeemable is gifted through the campaign, it will follow the campaign expiration rules. If the redeemable is gifted through Support Activity then it will follow the redeemable's expiration rules.