Subordinate bar numbers
Subordinate bar numbers are useful for organizing repeat endings, and for situations when the music is being altered, but the original bar numbers cannot be changed.
For example, you can use subordinate bar numbers to show where music has been added if a previous, shorter version has already been rehearsed. In this situation, players have likely started to associate certain parts of the piece with particular bar numbers, so if four bars need to be added after bar 10, they would be numbered 10a to 10d, after which the bar number continues from 11 exactly as it did before the new bars were added.
They might also be useful if you want different bar numbers for a repeat ending.
Subordinate bar numbers can be presented as either upper case letters or lower case letters.