By default, an Action is downloaded by ALL (MarvelClient) clients. The Who tab is used to target only a subset of users:
NOTE: Direct input and Access Definitions only supports multi-purpose and security groups, not mail only groups.
In case multiple Actions target the same users, you can either use (a) corresponding group(s) from the public adressbook, or create so called Access Definitions in the MarvelClient Configuration database for further re-use across multiple Actions.
NOTE: When checking ONLY FOR without selecting any Access Definitions and without supplying any direct input, an Action will only be deployed to users who have the role [Admin] in the MC Configuration database.