In Altium Designer, the L, M, N designators refer to the pad and footprint size. I believe these are from IPC-7351, but you need to pay for it.
- Least is 10% smaller than N; use for a really tight corner
- N is Nominal size
- Most is 10% larger than N; relaxed, good for hand assembly
All are compliant with the package size, so you can use any of them when laying out and the component will still fit.