Goodman Games

I'm currently on pg 140 of my initial read-through of the core rulebook and I saw mention of Mega Mutations but have no idea what is different about them. Can anyone please explain to me what these are and how you can get them as opposed to regular mutations?

Also, how do you know what die type to roll on the Fumble Table? It is by armor type, correct? It appears this way on page 125.

You get Mega Mutations by rolling 98-00 on Table 3:2 Mutations (pg 44). Table 3:3 Mega Mutations is directly below it on the same page.

Fumble dice per armor type is listed on pages 124-125.