What is it?
This customfield allows to select a user depending on its Permission on the Issue (here users having the Browse Permissions) :
How to configure them ?
When adding a new Customfield, select the Customfield Type User Picker By Permissions :
Then, you will be able to configure your customfield in order to filter the User by their Permissions.
The selection of the required Permission can be done in the following screen :
On this page: