What's meaning of %contact in this menu path?
All requests on admin/structure/contact/edit/[anything] will be handled by a Drupal form callback, contact_category_edit_form() in contact.admin.in` which returns a form array.

%contact means this parameter will be passed to contact_load() and the result will be pass to contact_category_edit_form() as first argument.

See hook_menu().


  Registered paths may also contain special "auto-loader" wildcard components in the form of '%mymodule_abc', where the '%' part means that this path component is a wildcard, and the 'mymodule_abc' part defines the prefix for a load function, which here would be named
  mymodule_abc_load().