reftex-index-default-tag

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

"idx"

Documentation:
Default index tag.
When working with multiple indexes, RefTeX queries for an index tag when
creating index entries or displaying a specific index. This variable controls
the default offered for these queries. The default can be selected with RET
during selection or completion. Valid values of this variable are:

nil Do not provide a default index
"tag" The default index tag given as a string, e.g. "idx".
last The last used index tag will be offered as default.

You can customize this variable.