Primary Characters
Primary Characters are central to the game’s theme, roleplay, or history. A Primary Character's main restraint is that there are some established events that we assume they were present for, and some established acts that they undertook. Small details may be changed, but to use an example: Batman cannot easily be played in a "Year One" style due to the vast number of other PCs who would require major alterations due to having trained with him, such as Dick Grayson, Jason Todd, Cassandra Cain, and so forth.
As with all things, this has some wiggle room. If you have a great pitch for how to take a Primary Character in an "early career" direction while leaving a narrative opening for others, then staff is willing to hear the pitch and to try and figure out a way to make things work.
Once a character is on the grid, they're 'locked in'. New applications have to consider the broad scope of previous roleplay and work in conjunction with the previous momentum.