Short answer
HubSpot's KB: action sets save a group of actions to reuse, such as Set marketing contact status plus an email send. They're only available in workflows of the same type. Data from a newly added associated source can't be used in the set.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Name sets clearly so the team picks the right one.
3. Related
Create task action and workflow settings.