definition
strict definition
address extension
The letter or suffix that distinguishes an address from others with the same house number.
An address extension is the part after the house number that tells two addresses apart: the a in 12a, or a similar suffix used where a plot was divided and the numbering could not be redone.
It is held separately from the number so that both can be read as what they are. Stored inside the number, 12a stops sorting and comparing as a number.
Practice differs by country. German addresses use letters freely, while others use fractions or a second number, and some do not extend numbers at all.