reftex-index-default-macro

reftex-index-default-macro is a variable defined in `reftex-vars.el'.
Its value is

(105 "idx")

Documentation:
The default index macro for M-x reftex-index-selection-or-word.
This is a list with (MACRO-KEY DEFAULT-TAG).

MACRO-KEY: Character identifying an index macro - see `reftex-index-macros'.
DEFAULT-TAG: This is the tag to be used if the macro requires a TAG argument.
When this is nil and a TAG is needed, RefTeX will ask for it.
When this is the empty string and the TAG argument of the index
macro is optional, the TAG argument will be omitted.

You can customize this variable.