It's always been a grey area even for so call purely residential. Strictly speaking, if it's residential, by right it's only for residential activities. Still people are use that to reply emails, work from home, calling clients from home, faxing from home. By right all this activities is prohibited in purely residential since it is clearly a business or commercial activities where taxes and utilities fees should be higher. Have u ever seen official raid into house due to this reasons?
Same case for soxo, it's going to be flexible use where no one can stop you putting a beds in your working place. Just the same as no one will stop you putting a fax machines at home.
Even some owners now are using third condo, house whatever purely residential title for airbnb. Does it legal? Strictly its not , cause you are delivering income from renting out as short term. Just like hotel, hostel which are operating as commercial. In fact this are violating the laws more than an office mix use as residential. Don't forget, soxo owners are paying commercial rates which is higher for taxes and utilities. So what's the fuss and issues if owners want to use it for mix use?
Those pure residential that use to do commercially activities is the one that violating the laws.