Sourcing

Sourcing is what gives a state a claim over a nonresident. Your home state taxes you because you live there; another state taxes you only if the income is sourced to it. For wages the default rule is where the work happened, which is why a remote worker who never enters the employer's state normally owes it nothing, and why an employer's mailing address is not by itself a tax nexus for its staff.

The convenience-of-the-employer rule is an exception to that default and the only one that matters at this scale. Independent contractors face the same underlying rule — personal-service income is sourced to where the services are performed — which is why a client's location does not create a filing obligation but a week spent working on the client's premises can.