From https://www.freebsd.org/doc/en/articles/committers-guide/article.html:
MFC to: If the commit should be merged to a subset of stable branches, specify the branch names.
MFC with: If the commit should be merged together with a previous one in a single MFC commit (for example, where this commit corrects a bug in the previous change), specify the corresponding revision number.
From https://www.freebsd.org/doc/en/articles/committers-guide/article.html:
MFC to:
If the commit should be merged to a subset of stable branches, specify the branch names.MFC with:
If the commit should be merged together with a previous one in a single MFC commit (for example, where this commit corrects a bug in the previous change), specify the corresponding revision number.