Two readings, one number that actually matters for safety. Here is which one tells you the water is sanitised, and how to interpret the gap.
Every chlorine test gives you free, total, or both. Free chlorine is the active ingredient, the part doing the sanitising. Total chlorine includes the chloramines, the by-products of chlorine after it has reacted with organics. The gap between the two is your single most useful diagnostic.
Free chlorine is the available, working chlorine that will kill the next bacterium that enters the water. Target band: 3 to 5 ppm in a hot tub.
Total chlorine is everything: free plus combined (chloramines). It is always equal to or higher than free.
Combined chlorine is the difference between the two. It is not a sanitiser. It is the smell, the eye sting, and the clue that the water needs a shock.
Combined chlorine should be no more than 0.5 ppm. If your free is 4.0 and your total is 4.3, you have 0.3 combined, and you are fine.
If your free is 4.0 and your total is 5.5, you have 1.5 combined, and you need a non-chlorine shock that day.
Most test strips show only total chlorine (often labelled simply as chlorine). To see the gap you need either a two-pad strip or a DPD drop kit, both inexpensive and worth keeping next to the tub.
Combined climbing slowly between sessions: normal. Shock weekly with non-chlorine oxidiser to reset.
Combined climbing fast within a single day: heavy bather load or low free chlorine. Both are addressed by a shock plus a small free chlorine top-up.
Combined high but free zero: chlorine demand episode. See the dedicated FAQ on that. Address the underlying cause before just dosing more.
Yes. The chemistry is identical. Pool DPD kits often have a wider range, which can be useful when you are diagnosing a high-combined episode.
Bromine has a similar split (free and total) but the gap is less diagnostically rich because bromine combined products are themselves partially active. Combined bromine up to 1 ppm above free is acceptable.