It has no real meaning other than extras are possible in addition to what you paid upfront.
Open minded can range from nude HJ in the last 15 minutes of an hour's massage, all the way to FS.

Problem is as discussed many times here, that ML become very expensive when you pay too much for a bit of extras only in the last part of your booking.

WL in all cases are better economy than ML. With WL, nude from the first minute and no negotiations required at more $$$$$$.