Nominal number
A nominal number is a series of digits that, unlike a number, does not indicate a quantity. It may even contain letters and symbols.
Explanation
They are often used to indicate a position within a hierarchy, as for house numbering. But sometimes there is not even an hierarchy as with telephone numbers.
Examples
A house number can be written as
12A
A telephone number can consist of
A flight number can be
B 3752 HL