What does bdy_width mean in wrfbdy_d01 file?

[YKim]

Hello,

I just wonder what does bdy_width mean in wrfbdy_d01 file.
In my wrfbdy_d01 file, bdy_width = 5.

float no2_BXS(Time, bdy_width, bottom_top, south_north) ;
no2_BXS:FieldType = 104 ;
no2_BXS:MemoryOrder = "XSZ" ;
no2_BXS:description = "NO2 mixing ratio" ;
no2_BXS:units = "ppmv" ;
no2_BXS:stagger = "" ;

Thanks.