I wonder how would one enforce this. I invoice once per month for the hours I logged (that might include vacation hours), but the invoice itself says "Software Development Services {month} {year}". Or do you mean we wouldn't be able to have vacation as part of the contract? Or not allow a contractor to work for one client for more than X time per year? Seems pretty tricky.

And how or who would enforce this for me, who is an Estonian citizen, Argentinian resident, and operates from under a USA based LLC as a non-resident USA company owner, working mostly for European start-ups? Which country would do the enforcing in this case?