You can use page variables and page text variables normally in fox forms.
You can also use them in templates, see Template Markup.

Some special page variables Fox creates:

this markup added to a target page will display the number of posts on the page, counting the string #foxbegin. Use #foxbegin# and #foxend# on the message template.
this will give a three digit random number, which Fox can check against an entry in a field named access, to require users entering a random code, as a measure against robot postings. To have fox checking for a match of accesscode and access-post, the admin needs to set $EnableAccessCode to true.