PianoMan Alkalinity used to be called ‘temporary hardness’, it’s the constituent in the water that will precipitate out as scale, if there is not enough of it the water can be corrosive.
Limescale is made from calcium and magnesium (total hardness of the water).
The relationship between alkalinity & total hardness is very varied from one water source to another and there is little link to TDS, TDS also includes organic compounds & bacteria if present.
Alkalinity is the most useful parameter to know.