Special services are denoted by letter prefixes in their route number:

Numbers are combined with a letter prefix depending on the event type.

Some car numbers do not have a letter prefix.

Hence, a unique registration mark without letter prefix could be found on two different types of vehicles.

A state may choose to use letter prefixes for all, some, or none of its state roads.

The Army also used the following one letter prefixes for a brief period of time in the 1960s:

The English letter prefix indicates the type of car.

Later, the letter prefix was dropped and numbers were increased by a certain number of thousands, as follows:

No production so no sales or Russian letter prefix.

They were numbered in a separate sequence with no letter prefix.