请问耦合变量的含义model couplings

hustlai

一直没搞明白Extrusion、Projection这两种model couplings的含义，不知道怎么用。但是感觉现在在做的一个东西很需要用到这两个东西，所以希望大家帮忙指点一下。
Extrusion. These operators—General Extrusion, Linear Extrusion, Boundary Similarity, and Identity Mapping—connect a source and a destination and take an expression as an argument. When it is evaluated at a point in the destination, its value is computed by evaluating the argument at a corresponding point in the source. 这句话是什么意思？
Projection. These coupling operators—General Projection and Linear Projection—evaluate a series of line or curve integrals on the source domain, where the line or curve positions depend on the positions of the evaluation points in the destination domain.还有这句的意思也不太明白。
希望各位帮忙解释一下