Foreign income often arrives already taxed, and the money you have to chase is the tax, not the fee.
When a client performs abroad, licenses a song into another territory, or collects foreign , two countries can have a claim on the same pound. Most territories tax income where it arises, so they grab their slice at source, before the money ever leaves the country. That deduction is withholding tax.
The home country then taxes the client on their worldwide income anyway. Left unmanaged, the same income is taxed twice: once abroad at source, once at home.
The system's answer is double-tax relief, usually a credit in the home return for the foreign tax already suffered. But relief is claimed, not assumed, and it is only as good as the paperwork behind it. The adviser's real job here is not the arithmetic. It is the certificate chain: getting the rate right at source, collecting the evidence, and matching it onto the home return so nothing is taxed twice and nothing is lost.
Note up front: every rate, treaty article and threshold in this module is illustrative. rates, treaty access and reliefs are jurisdiction-specific and change, verify each one against the actual treaty and current guidance.