The answer is when it is issued for parking on private land. The document itself can be strikingly similar, and I am sure this was deliberate on the part of the companies that issue notices on private land but the legal basis for the notice is completely different.
A parking ticket on a road is a penalty for committing a breach of parking law, either a byelaw imposed locally by a council or national law such as double yellow lines etc. If you don’t pay the penalty can be registered as a fine at the magistrates court and enforced.
A notice placed on a vehicle on private land can only be to tell the driver that they have breached a contract. The logic is this, the land owner erects a sign which says “if you pass this sign and park here you enter into a contract to pay £x for doing so” Enforcement is as a debt for breach of contract in the civil courts.
Up until October 2012 land owners had a problem, the contract could only be with the driver who should have seen the sign, but the only details they could get were for the registered keeper and they couldn’t prove the two were the same. As of October, possibly in “compensation” to land owners for making wheel clamping on private land illegal, the registered keeper became responsible for breach of parking “contracts” on private land closing the loophole.