The same set of lashes with the same mapping will look differently on everyone. Other than the amount of natural lashes a client has, the main reason for this is eye shapes. Round, almond, hooded, deep set, up turned, down turned it all makes a big difference. Below are the main types you will come across.
Generally a C curl is perfect for round eyes, possibly a CC as long as they aren't overly round. Anything over a D curl is going to lift the eyes too much and give the client a startled look. Looks amazing with a squirrel or cat eye map to widen those eyes.
The most easiest shape to work with and will suit most mapping styles. Although, you should take into consideration whether the eyes are up or down turned and the eyelid space when choosing mapping and length.
Up Turned Eyes
A C curl will give these lashes width, without adding too much lift (which is what they already have). If you wanted to bring the eye in you could do an open eye, doll look. If the eyes are extremely up turned avoid cat eye.
The line that starts from the inner corner is more or less even with the outer corner, This means that the eyes are neither up turned or down turned, which means just like almond eyes they will suit most mapping styles and curl.
The outer corner is below which determine that they are downturned eyes. CC or D curls will help to lift those outer lashes. Squirrel mapping would look amazing on these eyes as it would give them a lift with the longest length at the brow arch.
Hooded eye are eyes that are hidden from under the hood but still have some eyelid space visible. To complement the eyes and to hide the hood, you would need to use longer lengths. As the hood on some eyes is where the brows natural arch is. Squirrel styling would hide the hood.
Deep set eyes are eyes that have no or very little visible eyelid space. Just like hooded eyes, longer lengths would look best, but you could use any mapping as long as the lengths cover the lid space. If you wanted to try something different an L curl would look brilliant on both of these eyes as it would cover the lid space and the hood.