about edge flag

If i specify the edge flags of a mesh,would it get a more efficiency to GPU or just visibility-well?

what is visibility-well? I guess you are using some online translator.

Edge flag is for making edges visible or not.When you render in wireframe of course. It also works when you render in point mode.

Otherwise,how does opengl implement backface culling?specify the vertices wound?colockwise,counter-colockwise??I read about a way calculate the degree between nromal of face and view direction?!So,what is opengl option?!

Face culling is dependent on vertex order.
You can read how GL does it in the GL specification.