Measurments in modeling

What does finger to finger measure means?
what about navel to toe?
soulder to finger?
and for what do they need those measurements??

thanks

(:

Hey im 15, 5ft4 and i weigh 102lbs, i want to be an eye model cause i get told i have big eyes and long eyelashes, but i dont know wether there is a certain measurement for this type of modelling? And i dont have any idea where to begin ! x