I allways forget the term.
Its the polygon lightdistribution, that makes
the smooth lightshading.
(in contrast to flat shading)

If you have polygons that are attached on the same vertecies,
they share the lightditributuion, and the polygons
have a smooth lighttransition.
If you have two models, you will have a gab in the elumination,
as they are of course not attached polygons.

Best is to test that in the engine using dynamic light, and you will see what i mean.