Chromatographic response function, often abbreviated to CRF, is a coefficient which measures the quality of the separation in the result of a chromatography.
The CRF concept have been created during the development of separation optimization, to compare the quality  of many simulated or real chromatographic separations.  Many CRFs have been proposed and discussed.
In high performance liquid chromatography the CRF is calculated from various parameters of the peaks of solutes (like width, retention time, symmetry etc.) are considered into the calculation. In TLC the CRFs are based on the placement of the spots, measured as RF values.