a warning occured when running CST

sorry i can not insert Chinese words currently
when i was running CST in F-solver under unitcell boundary condition, with source type of Zmin and Zmax both excited, a warning occured saying:
" the exitation of the Floquet or periodic port modes at Zmin corresponds to the opposite direction asa compared to the scan angle definition in the boundary condition dialog. To excite a plane wave with the given scan angle at Zmin, please change the direction in the aforementioned dialog to "outward".
may i know the difference between "inward" and "outward"? as i give an exitation, it will surely transfer through the boundary i defined, does it really matter whether it is in or out?
and if i need set "outward" for Zmax, while "inward" for Zmin, how can I make it?
Thanks a lot~