The ideal PH is around 7.4 and the pool maintainer has to either add acid or alkaline to get to this level.
7.4 is considered a good compromise between Chlorine sterilisation and comfort on the eyes and mucous membranes (which - surprise surprise - is 7.4). This is quite difficult to do manually and is gauged using a color chart test kit.
Another important measurement is ORP (Oxidation-Reduction Potential) which in simple terms is an electronic measurement of the effectiveness of the Chlorine in the pool - and the recommended minimum reading for this is 650 Millivolts.
So, for a truly automated solution, measuring PH and ORP enables the system to dose acid/alkaline and chlorine (this includes turning on a salt water chlorinator at the right time and duration) in the right balance - for a perfect crystal clear pool.