What is TDS pen used for?

[Gareth tan]

Hi i was going to get a ph pen and saw this Total Disssolved Solids pen and im wondering what is it used for? Any one knows thks!!

[skytan]

https://en.wikipedia.org/wiki/Total_dissolved_solids

Normally planted tank this won't be a big issues but from what i heard some Shrimp keepers is very particular over this.
My tank TDS is 100-130 normally, my PH monitor have a seperate probe for TDS