its not a set, its more a combination of objects which you can use on a dynamic group.
In case of groups it searches for users and groups with the attributes you define for searching, but only generic attributes are shown, which are common to both object types.
You can also built something similar in situations where "All eligible resources" is not avaiable, just create one set for users and one for groups for example with ResourceID in xxx and create a third set to combine these to sets even with ResourceID in <Set1> and in <Set2>. Now you have one set with different objecttypes which you can use to maybe set permissions on.
This is for example used on the Sets for giving permissions to the objects of Keyword: BasicUI: All basic configuration objects.
all eligible resources is very limited you cannot modify anything on it just use it where it appears.