In my head, people may use the term head canon for their own content to differentiate between things published and imagined or between things set in stone or not set in stone.

An example that comes to mind is the d&d setting Eberron. There was an event called the mourning which set things into motion. There is no canonical explanation about what caused it. The creator has said there will never be a canonical explanation because they want DMs to be able to be flexible with it in their stories. So, to me, whatever the creator believes happened would be considered head canon because he specifically said there will never be a canonical explanation. I haven't seen the creator use the term head canon in this context, but it fits well I think.