I have a multi-select custom field, Affected Services, that shows locked in the Custom Field area of Admin.

This says that it's locked by Jira or a Plugin...I get the "locked by Jira" if it's a system field, but it's not.
How do I find out what is locking this field, how can I edit the context choices in the field?
I should also mention that this custom field was created for a Jira Service Management Project and is only used there.