reftex-index-section-letters

reftex-index-section-letters is a variable defined in `reftex-vars.el'.
Its value is

"ABCDEFGHIJKLMNOPQRSTUVWXYZ"

Documentation:
The letters which denote sections in the index.
Usually these are all capital letters. Don't use any downcase letters.
Order is not significant, the index will be sorted by whatever the sort
function thinks is correct.
In addition to these letters, RefTeX will create a group `!' which
contains all entries sorted below the lowest specified letter.
In the index buffer, pressing any of these capital letters or `!' will jump
to that section.

You can customize this variable.