recentf-filter-changer-alist

recentf-filter-changer-alist is a variable defined in `recentf.el'.
Its value is

((recentf-arrange-by-mode . "Grouped by Mode")
 (recentf-arrange-by-dir . "Grouped by Directory")
 (recentf-arrange-by-rule . "Grouped by Custom Rules"))

Documentation:
List of filters managed by `recentf-filter-changer'.
Each filter is defined by a pair (FUNCTION . LABEL), where FUNCTION is
the filter function, and LABEL is the menu item displayed to select
that filter.

You can customize this variable.