They can also be used as part of mixed additional member systems. (en.wikipedia.org)
The Assembly election uses a form of Additional Member System (AMS), with 14 local constituency seats (spanning two or three London boroughs) with winners elected by «first past the post» (or plurality rule) voting. (democraticaudit.com)
Any method of adding MPs to ensure rough proportionality is called additional member system (AMS). (channel4.com)