The general idea of the segment `STARTUP` is that there's only exactly one module placing content in that segment. Together with placing that segment in the linker config at the top of the segments filling a certain memory area this makes sure that the content in question is placed at the very beginning of memory area in question.

I understand that an Atari cart doesn't depend on that placement because of the explicit links to `cartinit` and `cartstart`in the cart "header". However for consistency reasons 'STARTUP' shouldn't be used nevertheless. I'd place the cart stuff just in the usual code segment.