Critical BCs question: choice of boundary conditions for Macro and micro channels?

Hi everyone,
I read this thesis ( in which the author gave in appendix the boundary conditions used for macro and micro channels. I noticed that the choice of boundary conditions is critical according to which case you are studiying : micro or macro channel. Why? Why is it different ? And why the boundary conditions in the micro channel case are “not common” ( use of Knudsen number ) ?
Can anyone explain this please?