There are three options to create a subset, all of them are equivalent:
Here you can setup a subset, delete it and see an overview of all the subsets in your project as well as the number of included references, but you cannot actually add a reference here.
After creating a subset in the reference detail, the reference is automatically added to this subset.
Here you can add individual references to subsets by clicking on the name of the subset in grey.
In the Subsets part of Bulk operations you can either add references to one of the subsets, or create a new one and add chosen references immediately. You can see the numbers of references already present in the subset (eg. 499 for “application”) as well as the number of references from your selection which are not present and therefore can be added (e.g. +476 for “application”, always in green colour).
It works in reverse as well: If you click on the red-coloured number, references will be removed from the subset (e.g. below, “application” subset: You have selected 975 references, 499 from them are already present in the subset, so you can remove them by clicking on -499 ).
