The subset helps to narrow down the results on the spreadsheet. It can be specify for any variable. Important thing is that applying subset is different than applying filter in the sense that it limits only the specified question instead of filtering out complete dataset cases.
Variable
Right-click anywhere on the variable to open the settings menu.
You might find it quicker to simply click on the filter icon which does the same job as "Edit subset". The filter is now inactive. Click on the icon to select values you wish to display. The filter color will change to orange when a subset of values gets created.
