when playing on homecart, you have the option of changing the AB for escaping from a slug - into D, for example,
this means you get rid of the mistakes you'd usually get by pressing AB together, and it makes it easier to get out of a slug.
the vulcan fix option 'sticks' the cannons on your slug at whatever angle they are at when you hold the button down, once again, it makes it very advantageous with the tank slug, as you can stick the cannons, keep firing by holding down the button, yet move the slug around too, it helps massively on the helicopter and submarine slugs too.
so .. we don't use those, they wouldn't be on a cab if you walked into an arcade and saw slug there, and you wanted to show off all hardcore like ...