This function has been renamed with the JWT 3.0 release.
Find the new documentation at:
Purpose
This condition is used for showing a transition only to those users in a field. This condition supports any User or Multi-User field or custom field.
Also text fields can be used. In these cases it's expected that fields contain the name of a user or a comma separated list of user names. Don't confuse user name with users's full name.
Example: Limit transition visibility only to current issue watchers
Once configured, condition will look like this: