I always tought that the term “farm” should be associated with a single XWiki instance where multiple subwikis are created.
This probably comes from reading Multi-tenancy (XWiki.org) during installation.
I remember having seen the same term used for another setup (several XWiki instances running on the same server), but I can’t find where, so I cannot be sure; it could have been in Wiki hosting service - Wikipedia.
I just found this comment on an old thread: What does "on farm" mean? - #4 by vmassol where he mentions independent wikis.
Moreover, when addressing some issues while installing some extensions, I have been told not to use Install on farm Numbered Content extension installation - #17 by tmortagne
So this is a request for clarification about the term on farm and how extension installation should be carried over when we have a single XWiki installation with several subwikis - and extensions should be installed on each of them, both for already existing subwikis and for those created after an extension has been installed (if that’s possible of course).