TDS is Total Disolved Solids. It is probably the x500 scale. Quick math will easily verify that. The probe measures conductivity and the meter caluates it to what unit you want. On mine there's EC(mS/cm), CF(ECx10), PPM500, PPM700.
EC is easy but some nutrient companies will give you a target ppm. You just have to make sure you know which scale they're using. It also has the benefit of being slightly more precise on these meters. I don't know why they don't take the EC reading out to the hundreds decimal but you can dial in to a greater detail using ppm. I've found it isn't worth it. .1EC is 50ppm on the 500 scale. That's such a small window, I don't really care.
Good luck on your grow.