A "main" arrangement is the arrangement a Numberblock makes the most in occasions that doesn't require them to be in other arrangements. It doesn't have to be episodes, sometimes main arrangements are confirmed by games and books (such as 55). Main arrangements are "overall", that means if a character has a different main arrangement, it still doesn't count.
For example, 24's main arrangement might be 4x6 in Numberblocks World, but it is clearly 3x8 everywhere else, so the 4x6 basically DOESN'T COUNT. And 55? In "One Giant Step Squad", the only reason he's in a step squad arrangement or a "Five Tens and Five" arrangement is because HE WAS MADE THAT WAY, the situation asked for it. Same thing with the rocket ship, so they all don't really count. What we need is a situation who doesn't have any requirements for 55's arrangement. And bam, in Numberblocks World, 55's "icon" is in the rocket ship shape. Then in Numberblocks Annual 2022, 55's image is also in a rocket ship shape, and it's in the same place where the main arrangement is shown in other pages. Therefore, 55's main arrangement is the ROCKET SHIP.