From what I understand:
- Early halachic compilations (throughout the Geonim period) sourced directly from Talmud Bavli
- Talmud Bavli often sources opinions by actions and text in the Torah, and oral Halacha passed down from Moses
- Later Halachic compilations (Shulchan Aruch), sources from earlier halachic sources
- The Mishnah Brurah is a halachic commentary on Shulchan Aruch
Is everything written in later Halacha either Rabbinic/Torah prohibitions (whether written or oral), where it isn't qualified with "someone who is a G-d fearing man..." and the like? Or does it include customs as well? How can we know which is which when reading halachic sources?
As an example, there are cases discussing "new" phenomena such as hat-wearing discussed here, where the Mishnah Brurah clearly states a "must" that does not seem to be a Rabbinic/Torah prohibition.