It affects you if you work in construction and supply another VAT-registered business that is CIS-registered and is not the end user. Under the reverse charge you do not charge VAT on your invoice; instead the customer accounts for it. Your invoice says the reverse charge applies and states that the customer must account for the VAT. It exists to stop missing trader fraud, and it has a significant side effect: subcontractors who previously enjoyed holding VAT until quarter end lose that cash flow, and many end up in a repayment position, which is a good reason to consider monthly returns. The end user question is the one that causes disputes — if your customer is the final consumer of the work, normal VAT applies and you should get their confirmation in writing.