I recently received a question regarding site and system master pages in SharePoint. It seems that the purpose of these two master pages is not clear to everyone so here is a short summary:
By default the following rule applies:
- Site Master Pages: used by all publishing pages - and only by publishing pages
- System Master Pages: used by everything else including forms and view pages
You cannot change the default behavior - but you can use SharePoint Designer to change the master page assignment for individual pages if required.
If you open a page in SharePoint Designer you can see a MasterPageFile setting which can be modified:
- "~masterurl/default.master": the page will use the system master page
- "~masterurl/custom.master": the page will use the site master page