Feature license assignment on users
Key Takeaway
Every user has exactly one user license. To give a user a feature their user license doesn't include, select the matching feature license checkbox on the user record. You can select more than one, but you can only assign feature licenses your company has purchased.
A user license sets the base set of features a user can access. A feature license is an add-on that gives a user one extra feature their user license doesn't include, such as Marketing User for campaigns or Service Cloud User for the Salesforce Console for Service. When a user needs a feature their user license doesn't include, you need to know which feature license unlocks it, whether your company has purchased it, and how to assign it on the user record.
How licenses and permissions fit together
Every user must have exactly one user license. You can add one or more feature licenses or permission set licenses on top of the user license to unlock more permissions step by step.
Feature licenses and permission set licenses are separate license types. Both add access that the user license doesn't include. You can assign a user any number of either type.
Think of each permission as a lock and each license as a ring of keys. You can give a user a permission only if one of their licenses holds the key to that lock. The user license is the ring every user has, and a feature license adds a key that the user license is missing.
You get an error message if you try to give a user a permission that none of their licenses supports.
For example, the Salesforce user license includes Read on contracts, but the Chatter Free user license doesn't. If you try to give a Chatter Free user Read on contracts, the assignment fails.