ppm is always mass per mass, unless SPECIFICALLY indicated otherwise.
So . . 1000ppm means: 1000 mg per 1kg of the total.

Are you planning to mix up your own nutrient solution from the component salts? If so, then weighing out amounts per a certain weight of water is what you will need to do.

If you are trying to figure out the right ppm you should strive for with your pre mixed nutrient solution as read by your TDS meter, then the ppm becomes rather useless since you need to know which ppm estimation scale they use for the meter as well as for the information saying what the recommended ppm should be.

This is because judging ppm from an EC meter is really just an estimate based on the conductivity reading for one of the salts in the mix. If you are just looking for info about recommended nutrient levels and how to read/test for them. I say go for EC or CF readings as they take a little of the guess work out of it.

Here is a link that might be useful: My Hydro page