That's unfortunately not our's to decide. Only prices are delivered by all brokers, any other data is up to the broker. If they deliver tick frequency instead of volume, marketVol returns the tick frequency. If they deliver spread, marketVal returns the spread. If they deliver nothing, the functions return 0. Different function names or bigger structs won't help.

For this reason, it makes no sense to guarantee a certain volume format. The only requirement is that marketVol returns at least some proxy of volume. Since almost all volume based indicators only use volume changes and not absolute volume, they will work. How useful such indicators are with a MT4 connection where volume is normally the tick frequency, is a different question.