下句（见补充说明）的从句部分是什么语法结构?Thereflectionprinciplesapplyevenwhensurfacesarecurvedaslongasthecurvatureofthesurfaceissmooth,thatis,aslongasthereisaplanetangenttothesurfaceatthepointof

Question

下句（见补充说明）的从句部分是什么语法结构?
The reflection principles apply even when surfaces are curved as long as the curvature of the surface is smooth,that is,as long as there is a plane tangent to the surface at the point of contact.
【译句：即使（反射）面是弯曲的（曲面）,只要面的弯曲部分是光滑的,亦即只要存在一个平面在反射点处与此曲面相切,反射定律也可适用.】
注：此句的中文意思我能理解,但不知从语法角度怎么分析?其中的那个that引导主语从句吗?

Answer 1

that is,:在这个句子中是插入语,用来连接后面的并列句.类似于：or