The China treaty was signed on 23 January 2006 and entered into force on 1 October 2006. The Hong Kong treaty was signed on 24 August 2017 and entered into force on 1 September 2018. ZATCA publishes both texts. A Cayman or BVI holding cannot rely on either, because Saudi Arabia has no treaty with those territories.
Saudi domestic withholding on payments to non-residents is 5% on dividends, 5% on interest and 15% on royalties. Under the China treaty, royalties are capped at 10%. Under the Hong Kong treaty, interest is not taxed and royalties are capped at 5% for equipment rental and 8% otherwise. Treaty relief is claimed online with ZATCA using form Q7B.
The Saudi company's profit is taxed at 20% on the share held by non-Saudi shareholders. Whether a Hong Kong company owned by mainland shareholders qualifies as beneficial owner under a treaty is a question for a tax adviser; Tasis is not one.
VAT is 15%. Registration becomes mandatory once taxable supplies exceed SAR 375,000 over the last 12 months, or are expected to over the next 12. An EPC or trading company that signs its first contract quickly often crosses that test at the start. See our VAT in Saudi Arabia guide.