As others have said, these are screen reader commands, but many are
standardized across screen readers. They do act differently in
different screen readers however. For example, System Access
acknowledges that there are landmarks on a page by automatically
starting to read from a main landmark, but has no way to navigate by
landmark. What NVDA and JAWS call combo boxes accessed with the
command c, System Access calls drop-down lists, accessed with the
command d. Another difference, and this one is kind of annoying, is
that System Access and NVDA both inform you that there aren't anymore
headings on a page but keep you at the last heading, while JAWS rapps
back to the top of the page. Different screen readers do have
different commands. For instance, in JAWS, insert F5 provides a list
of form controls, insert F6 provides a list of headings, and insert F7
provides a list of links. In NVDA, insert F7 brings up an elements
list, from which all the above elements can be listed by navigating to
the radio buttons and choosing the one you want.