The ONLY FOR direct input field allows you to enter person-, group- or certifier names so as to only target certain user(groups):
Examples:
Mary Miller/ACME
Groupname 123
*/Sales/ACME
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 addressbook in the direct input field, or create so called Access Definitions in the MC 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.