Does CDO treat longitudes as cyclic (periodic) or as fixed boundaries when performing remapping?

When performing remapbil (or other remapping algorithms), interpolation often relies on adjacent grid points on both sides of a target location. Near the longitudinal boundary (e.g., at −180°), does CDO treat longitude as cyclic and use values at 180°, or does it treat the boundary as fixed, resulting in one-sided interpolation?