LMR® 600 is a thicker cable than the LMR® 400.
You typically use LMR® 400 when going runs from 5ft to 40ft.
Anything 50ft and above, you want to use LMR® 600.
Because the cables will add loss to your signal, using the thicker LMR® 600 will reduce the loss when reaching those longer cable lengths.
LMR® is a registered trademark of Times Microwave Systems