The VAT domestic reverse charge applies to construction services between VAT-registered businesses where the recipient is CIS-registered and is not the end user. The supplier does not charge VAT; the customer accounts for it instead. The two schemes overlap but are not the same — CIS is about income tax deducted from labour, the reverse charge is about VAT — and a supply can be within one and not the other. The practical effect on subcontractors is a cash flow hit: you no longer hold VAT between invoicing and your return, and many find themselves in a permanent repayment position, which is a good reason to switch to monthly VAT returns. The recurring dispute is whether your customer is an end user, and you want their confirmation in writing.