A blanket determination is where a medium or large end client applies a single status conclusion, almost always "inside IR35", to a whole category of contractors or roles without assessing each engagement individually. It became common around the April 2021 extension of the off-payroll rules to the private sector, as risk-averse clients sought to avoid the cost of getting determinations wrong.
The firm's stance is that a genuine blanket determination is bad practice and legally risky for the client. A Status Determination Statement must be reached with reasonable care on the facts of the individual engagement. A determination issued across a category of roles without that individual assessment is very likely to be a failure of reasonable care, which can invalidate the SDS and move the deemed-employer liability to the client.
A contractor faced with a blanket determination is not without recourse. They can use the client-led disagreement process to make representations and require an individual assessment, with the client obliged to respond within 45 days. The careful distinction is between a true blanket policy (risky) and a client that has assessed genuinely similar roles consistently after proper consideration (defensible).