Is the 616 the main universe in all the Marvel multiverse or just the one Marvel focuses on?

This question's been bothering me for a while. I would like to know if 616 is like the ''head universe'' or just some random Universe Marvel chose to focus on. I know Stan Lee created 616 but since his time, there have been a boat ton of Universes. Earth-1610 AKA Ultimate Marvel is another Universe that is still being written. So what do you guys think?

The 616 is the main focus universe that is used to show differences between other universes like the UU. There is a universe that is very much like ours if i remember its the earth 0000 universe were all the marvel heroes are just comic book characters. The 616 does have a special case of being visited by many other parallel universes so you could say it could be considered the head universe.