There are three options how to create a subset, all of them are equivalent:
Here you can setup a subset, delete it and have a overview of all subsets in your project and number of included references, but you cannot actually add a reference here.
After creating the subset in 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 (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 references immediately. You can see the numbers of references already present in the subset (eg. 499 for “application”) and 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 reversely as well: if you click on red coloured number, it will be deleted 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 delete them by clicking on -499 )