XF 2.2 "+" sign in route matching syntax meaning? :+int vs :int what's the difference?

[Earl]

I see some of XenForo core route formats has this patten
comments/:int<profile_post_comment_id>/

and some of them has this + sign,
:+int<thread_id,title>/post-:+int_p<post_id>

What's the difference?
And where should I look if have to find how these syntax parsing so I could understand?

 

[Mike]

By default, parameters in a route are actually optional matches. This can allow matching of basic action-only components. (Like a route for threads/:int<thread_id,title>/ would still match threads/latest and assign "latest" as the action.

The + makes these parameters mandatory.