[const.iterators.types]

basic_const_iterator<Iterator>::iterator_concept is defined as follows:

(1.1)
If Iterator models contiguous_iterator, then iterator_concept denotes contiguous_iterator_tag.
(1.2)
Otherwise, if Iterator models random_access_iterator, then iterator_concept denotes random_access_iterator_tag.
(1.3)
Otherwise, if Iterator models bidirectional_iterator, then iterator_concept denotes bidirectional_iterator_tag.
(1.4)
Otherwise, if Iterator models forward_iterator, then iterator_concept denotes forward_iterator_tag.
(1.5)
Otherwise, iterator_concept denotes input_iterator_tag.

The member typedef-name iterator_category is defined if and only if Iterator models forward_iterator .

In that case, basic_const_iterator<Iterator>::iterator_category denotes the type iterator_traits<Iterator>::iterator_category.