BOUNDARY(ng)%ubar_west(j)=my_flux/my_area
BOUNDARY(ng)%vbar_west(j)=0.0_r8
This is standard Fortran 90 syntax for accessing user-defined types. BOUNDARY is an array of an object, where BOUNDARY(ng) is referencing just one element of that array. The % access an element within that object, say zeta_west. So the whole BOUNDARY(ng) contains boundary conditions for all sides, all variables for one grid.