I think this is a bug, at least unexpected. I have these words (and others) in a document:
23rd Regiment
19th Mercantile
The index lists 23rd Regiment under the alphabetical letter "R" taken from the "rd" ordinal ending. I would expect it to be listed alphabetically under the characters "23".
The index lists 19th Mercantile under the alphabetical letter "T" taken from the "th" ordinal ending. I would expect it to be listed alphabetically under the characters "19".
The indexed words are, however, sorted before the alphabetical letter "A" - so it's sorting by the two numbers "19" and "23" but listing them under the ordinal's first letter instead of the numbers.
copied from index:
T
19th Mercantile 225
R
23rd Regiment 225
A
Air 225
B
Bird 225