not having read the regulations (only heard of bits and pieces), and not having setup a home business in HDB myself...
i think one of the essential thing is that the neighbours are not disturbed in the process. like no operation of heavy machinery - can imagine the ruckus in a HDB flat!
there's a backpackers hostel that's setup right in a HDB shophouse (2nd storey) in woodlands. One of the criteria the owner had to meet was that whoever has to visit the business don't need to go through a common corridor to get to it.
eg. for the backpackers, it's not very nice if you have 10 or 20 backpackers walking in and out of the premises, and disturb the neighbours - how so? well, you never know what's your neighbours' threshold of tolerance. or say a clumsy backpacker manages to send a neighbour's potted plant crashing with a super-sized duffle bag...
the backpackers hostel's entrance is the back door of the shophouse, so there's no problems there.
for the studio, i guess one thing is whether people might become noisy and disturb the neighbours, whether if any heavy human trafficflow that may arise will disturb the neighbours, and so forth.
people have been having home studios in HDB flats so i don't think doing paid work in it is a problem in so far you meet the regulations and not disturb the neighbours in the process.