Registration marks are not items of property in their own right, so it is not possible to acquire legal title to them. They are assigned to, and may be withdrawn from, vehicles rather than keepers, by the Secretary of State, as part of the basic registration and licensing process required by law. The registration mark is a unique means of identifying a vehicle, primarily for taxation and law enforcement purposes. It is assigned to a vehicle, and it normally remains with that vehicle until it is broken up, destroyed or sent permanently abroad. However, to meet the widespread interest in personalised and cherished registration marks, the Agency provides special facilities to allow motorists to acquire and retain the use of particular registration marks.

No guidelines to relevant personnel on the avoidance of allowing swear words or rude phrases to appear on registration plates have been produced. However the Agency has set up a propriety steering group, which meet at least bi-annually to continually review the appropriateness of registration marks as society changes. The Agency's policy in respect of the withholding of registration marks from either normal issue or from its sales scheme is clear. They are withheld in instances where if displayed on a number plate they are likely to cause general offence or embarrassment. The reasons can be on the grounds of political, racial and religious sensitivities or simply that they are regarded as being in poor taste.