Why are there more names in my User List field than just the licensed users in my tenant?

[Shaun McGuire]

I created  custom field of type User Picker (single user). I expected this list to be limited to just the names of users that I have assigned access to (namely our employees and a handful of outside consultants).

Instead, there are hundreds of external user names in the list as well. I don't know where they came from. I am wondering if they are somehow Jira Service Management Customer users?

How can I have a custom field that excludes customer users who don't have a license or assigned access to my Jira tenant.

[Mikael Sandberg]

This is expected, and your thinking is correct, the user list also contains your external customers. In order to limit the list to only certain users you can use User filtering for the custom field and either limit it to a specific group or project role.