Cl is the chemical name for the element chlorine. The molecule we use is Cl2, 2 atoms of chlorine combined. We refer to the states of the chlorine molecule in the pool water as FC, free chlorine, which is available to actively sanitize, and CC, combined chlorine, which is bound up with nitrogen (N), as chloramines, and is not available to sanitize.
CH stands for Calcium Hardness. The chemical name for calcium is Ca.