c="n" in selectors will select n entites.

n>0 will give the n closest
n<0 will give the (-)n furthest
n=0 will give all entites in oder as if n>0 (same as if you don't use it al all)

However, when you use c=1 this is not quiet consistent with any other value.

c=1 for some reason prefers the command-caster even if you execute it from a different position.

How to reproduce:
1) summon two ArmorStands with CustomName "A" and "B" some blocks apart from each other
2) use "execute @e[name=A] ~x ~ ~z say @e[c=n]", so that the execute results in the position of B
3) when you use n=2 it will say it in correct order. First B, then A (or the player if it is closer to B than A) however, if you use c=1 it will print A instead of B, which is not consistent with the other result.

Same happens whe using @p (which has a built-in c=1):
1) Have a player stand some block beside you
2) use "execute @p ~x ~ ~z say @p[c=n]", so that the execute results in the position of the other player
3) again, when you use c=2 it will print the other player first and then either yourself or another player that is closer to the player you targeted with the execute. However not using the c (which is equal to c=1) will print yourself even though the other player is closer to the offset execute position