meaning of p in %pa[p] in the printk formats

The question is with regard to the printk formats. The documentation that I'm reading is located here

Here is an excerpt that I'm asking about:

Physical addresses types phys_addr_t:

    %pa[p]  0x01234567 or 0x0123456789abcdef

    For printing a phys_addr_t type (and its derivatives, such as
    resource_size_t) which can vary based on build options, regardless of
    the width of the CPU data path. Passed by reference.

What does [p] in %pa[p] stand for? What does it mean?