recentf-arrange-by-rule-subfilter

recentf-arrange-by-rule-subfilter is a variable defined in `recentf.el'.
Its value is

nil

Documentation:
Function called by a rule based filter to filter sub-menu elements.
A nil value means no filter. See also `recentf-menu-filter'.
You can't use another rule based filter here.

You can customize this variable.