In this video watch from 15:48 to 19:00
The theory is sodium levels when using 2 part dosing can result in an excess of sodium chloride and not enough of the other elements in the water over time that water changes will not necessarily correct. Dosing a ABC 2 part, kalk, or using a CaRx will. Additionally, sodium chloride should be about 70% of your total salinity.
At least this is my basic understanding and reasoning for measuring Sodium.