basicly size .
a #10 will not fit correctly on a #11 nipple . this can cause double falls or even splitting of the cap .
a # 11 will work on a # 10 nipple but it easly falls off .
normaly # 10 nipples and caps are used on revolvers and such .
now notice i said normaly . this is becouse a rifle can use a # 10 nipple depending on the geomitry of the lock AND i have know folks who have modified #11 nipples to take #10 caps