Other than fabric, please list the features of a pair of typical orphan suit trousers which differentiate them from ordinary trousers. Let's use a BB 1818 suit as a starting point since most would agree that is a typical suit. Don't list outlier features like uncommon waistbands, lack of belt loops, etc.
You've pretty much answered your own question. The fabric and pattern of suit trousers will often be different from standard trousers.