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 ?
Create the Custom Field
When adding a new Custom Field, select the custom field Type User Picker By Permissions :
Configure the Custom Field ...
You will have to configure your custom field in order ...
... to filter the User by their Permissions.
The selection of the required Permission can be done in the following screen :