Difference footprints of part in Altium Designer
I think they stand for

N-nominal,
M-maximum,
L-least

Not sure about V though.

Also if you look closely, for example, at SMD0805's N, M and L variants, you will find that they increase in size, just by little.

About why are they used when there isn't much of a difference in their sizes, read this: 
https://blogs.mentor.com/tom-hausherr/blog/2011/01/28/pcb-design-perfection-starts-in-the-cad-library-part-12/