"Animal skin treated" - what does "treated" mean in this context?

Solution 1:

"Treated" means processed in this context, here's a relevant quote from wikipedia:

To make the parchment more aesthetically pleasing or more suitable for the scribes, special treatments were used. According to Reed there were a variety of these treatments. Rubbing pumice powder into the flesh side of parchment while it was still wet on the frame was used to make it smooth and to modify the surface to enable inks to penetrate more deeply.

Treatment is basically the process of changing some substance properties by mechanical and/or chemical means. Like in, for instance, water treatment or surface treatment.