Interesting topic.
So let’s say a business currently has two trailers (one which it owns and another hire one). The company has two tractor units. It intends to buy a third trailer of its own, but only has two trailers on the O licence. Because there are only two tractor units it is impossible to operate three trailers at any one time.
Question is, are we allowed to buy a third trailer and then VOR it whenever we don’t use it or just change them?