"The company covers the room and the tax, the guest covers everything else" is an ordinary corporate arrangement and a tedious one to apply by hand across a week-long stay.
A routing instruction is a standing rule on the folio: send this charge type, or this source, or this transaction code, to that window. Rules are matched most-specific-first, so an operator can write "everything to window two, except minibar to window one" and get exactly that without reasoning about priority numbers.
The rule is evaluated when the charge posts, not when the bill is presented — and it is deliberately not retroactive. Adding a rule does not move charges already on the bill, and withdrawing one does not move them back, because re-attributing an already-invoiced line would put a window's fiscal document out of agreement with the charges it quotes.
A charge that matches no rule falls to window one. An unrouted charge is the guest's until somebody says otherwise.