The polarization of the light source can greatly impact the efficiency of the grating. If no polarization is marked next to an optic, it is tuned for maximum efficiency for unpolarized light. However, some gratings will be marked with maximum efficiency for either P-polarized or S-polarized.
P-polarization, also known as TE polarization, is when the incident light is polarized parallel to the grating grooves.
S-polarization, also known as TM polarization, is when the incident light is polarized perpendicular to the grating grooves.