Parts per million,used to measure nutrient strength,and is vital when growing DWC method,no prob glad to help,I have a book called How to Hydroponics it show you how to build systems and gives you general idea on hydroponics.
Someone said to use the rockwool after you germinate, but I have just recently planted 3 new sprouts and they rooted twice as fast as my last in the rockwool and a much stronger wet of roots.
it should work as long as theres no additives in it...
anything would work as long as its clean..
even marbles!
ive seen people use chunky perlite before too.