The User ID Assignment gives you perfect control over who exactly is going to be able to see your box. The only information that you need to know is naturally the User ID which you can see from Joomla's User list.
In the table below you can see which options are available to setup the User ID Assignment.
The User IDs can be typed in manually using a comma separated list.
For instance if my User ID was 1 and another User's was 2 then all I would need to type is 1,2 to display the box only to me and that particular User.
A very common use case for the User ID Assignment is when you want to display a particular message to one or more specific users who may have won a contest in your Joomla website and you'd like to congratulate them or award them their gift.
The Joomla User ID can be retrieved by navigating to Users > Manage in Joomla's Administrator Panel. You then will be redirected to a screen from which you can see the User IDs at the far right of the screen as shown in the screenshot below.