BS 5839-1 vs BS 5839-6 — who uses what, and when?

Seeing this one come up repeatedly. The short version: -1 for commercial and non-domestic premises, -6 for dwellings (domestic). But there are grey areas: - HMOs at different scales - Care homes and sheltered housing - Student accommodation - Live-work units What is your rule of thumb for boundary cases? And which grade or category do you typically specify for the most common residential types?