Thank you. After looking around a bit more, I think I finally got the hang of it. This HZMB bus is the cheap option; as arriving pax, need to clear immigration, enter HK, collect checked bag, take local bus to the port to board this bus. The more expensive bonded bus skips all that.

Both bus companies say allow 1 bag. I will have 1 checked bag + a carry on + a personal item. The carry-on would be either a backpack or a roller bag. The personal item would be either a backpack or laptop bag. Anyone know how they treat these?