Форуми / Cotonti / Support / Rewrite URLs

I've been fiddling around with the rewriting system. I've copied some of the rules from Cotonti.com but I am getting this error message:

Rule contains callbacks: {username_url_structure()}
Rule contains callbacks: {page_url_structure()}
Rule contains callbacks: {list_url_structure()}
You will have to add rewrite options for them manually.

It would be nice if someone could explain what this 'callback' is exactly, because I don't really understand what is meant by this.
Also, when I visit index.php, I get a fatal error:

Fatal error: Call to undefined function list_url_structure() in 
/home/db045b0018/domains/ghengeveld.nl/public_html/squadraat/Squadraat_Cotonti/system/functions.php 
on line 4341