what is the difference between a credible witness and a subcribing witness when doing a notarizing in Calif?

Answer 1

A "subscribing" witness is one that sub-scribes = literally "writes underneath" -- meaning, the person signs at the bottom, below whatever they are witnessing.

A "credible" witness is one that is competent to understand the proceedings, and understands what they are witnessing.

Generally, someone must be credible to be a witness --- whether they sub-scribe or sign in some other place on the document is a matter of the format of that particular document.