Its all very easy..... do it this way and you will never screw it up.
If you buy anything for them.... insurance... a bottle of window cleaner.... a rag... anything at all besides a friendly bottle of Dr. Pepper.... they are your employee so you must treat them as one. Do you train them? They are your employee. Do you tell them they can't get their own jobs or work for another cleaning service? They are your employee.
If they buy all their own stuff and you give them almost no direction except what job to do on what day... and they might have other jobs working for other cleaning companies or for their self they are your sub contractor. And yes, they must have their own insurance and all that crap.
That's the easy way to remember, not the official way.