My nutradip trimeters sucks, Don't buy one.
I wish I had not.
The tds portion failed within 2 months.
Go American, for reliability,
and so you will have a meter that works.
You can buy two milwaukee meters for the price of a trimeter
and the american meters work.
To answer your question.
TDS(total dissolved solids) can be express as E.C. and PPM
and are all the same measurement expressed in different scales.
E.C. is expressed in micro siemens.
I like ppm (no Math) plain and simple, parts per million.