Also Pl clarify "tangent continuity" and "curvature continuity" and its applications.
Thanks in advance.
extrapolate in catia allows you to extend the surface along the boundary curves which you determine.
And in modeling there is some continuities called as G0, G1 and G2. these mean that. G0 is point continuity, G1 is tangent and G2 is curvature. They are used in connections between surfaces and curves. If you use blend command to connect two surface, you can see differences between them easly.