Default [Q] GSM and LTE modem difference?


I have a i9505 s4 mobile. When I change modem I always use modems from a tread here in XDA developers and flash them with odin.

But I am wondering about the difference between GSM and LTE modem.

If I flash the LTE modem, does only the 4G/3G LTE modem gets updated and not the GSM?
And if I flash only GSM does the same happens visa versa?

Should I update both modems?

I always switch between GSM and 3G when im in different areas to make my mobile last longer...