I guess you mean the wp_page_menu
fallback, where you can use the filters wp_page_menu_args
and wp_page_menu
to modify it’s output.
Notice that this function is a wrapper for the wp_list_pages()
function, that ships with the wp_list_pages_excludes
and the wp_list_pages
filters.
That function is again wrapper for the walk_page_tree()
function, that activates the Walker_Page
class walker by default. This class extends the Walker
class.
Here’s a little schematic overview:
wp_nav_menu()
\
\__ wp_page_menu() # fallback
\
\__ wp_list_pages()
\
\__ walk_page_tree()
\
\__ new Walker_Page # default walker
\
\__ extends Walker
So there are few ways to modify the output from wp_page_menu()
.