Its points.

Its not real intuitive at the beginning.  Say you find a lost library for +20 point research.  Its the same thing when your trading.  Your giving them x number of points towards their research and you are getting x in return.  Even if they are in the same branch of the tree.  Ill edit this as I think of more.  Thats a quick answer.

so ill give you 20 in civics for 20 of yours in civics.  even though the AI is way ahead of you in civics it just give you both a +20 in civics.  No subtraction (thats the main thing to get. No subtracting is done)

Early tech can be completed with a +20.  late techs take hundreds of points.

Side note.  As of yet, the trade doesn't seem to go into the exact tech you are researching, it just finds a home in that tree somewhere.  If you mess with it you'll understand.  Hard to explain.