User license types and their capabilities
Key Takeaway
Every user needs exactly one user license, and it sets the baseline of features they can use. Pick it by matching the person's job and whether they are inside or outside the company.
A user license is what gives a person access to Salesforce, and different license types fit different people. Internal users may need the full CRM or only custom apps. Other license types fit external customers and partners, Chatter-only collaborators, and system integrations.
What a user license does
Every user needs a user license to access Salesforce, and each user can have only one. You assign the user license as part of creating a new user.
The license sets a baseline of features. Profiles and user permissions then adjust access within that baseline. For example, they can let some users only read records while others also create and delete them.
You add functionality for a user with permission set licenses, feature licenses, or usage-based entitlements, not with a second user license.
Changing a user's license also removes every permission set and permission set license currently assigned to that user.