The mass (weight) of the liquid stationary phase or the active solid in the column. The mass (weight) of any solid support is not included. In the case of partition chromatography with a liquid stationary phase it is identical to the liquid phase mass (weight).
Source:
PAC, 1993, 65, 819. 'Nomenclature for chromatography (IUPAC Recommendations 1993)' on page 833 (https://doi.org/10.1351/pac199365040819)