In the United Kingdom at some larger railway station platforms are split into sections.
It was introduced to enable multiple trains to use the platform at the same time.
Depending on the station the platform may have a letter suffix (such as Birmingham New Street station) or may actually be split numerically (Bristol Temple Meads has platforms that are identified with two numbers. Eg 3 at one end, 4 at the other) the designations are so that people then get on the correct train.
Historically trains were much longer in the uk. 12 or 14 carriages long. But many trains in the UK today are much shorter, so this enables better usage.
In addition it's worth noting that some of the U.K. Operating companies also have colour zones on the stations they use (this seems to be diminishing but is worth mentioning) These were introduced so you can identify the section of the platform to wait for your correct carriage. (I.e. Carriages A and B will be ajacenent to the red zone)
To answer the question. In the UK it seems to be allocated arbitrarily.