as far as i know, the main difference in the couture items would be,
- more hand finishing. tho i have heard its not all that much. their website however, implies it is a lot. go figure.
- as well, i understand the fabrics they use for couture are a step above. a cashmere mainline SC, and a cashmere couture SC, for example, would not be made from the same roll. within fabrics there are varying levels of better grades
- some degree of better construction, not sure how true that is, as i understand mainline zegna is made pretty darn well. i have a decent amount of mainline zegna items, and they are all fantastically made.
- other odds and ends, like buttons, and button hole shapes, just small finishing items to differentiate the line.