So what we need is a documentation of how to query UPAs (Universal Product Attributes). Could you please help us out?
Thank you in advance.
The only reference I found for UPA is related to ATPCO and, as far as I know, Sabre does not offer ATPCO content directly. Data goes from ATPCO mainly for shopping and pricing stuff.
If you are new to Sabre, you might want to try checking the Getting started with Sabre APIs and then check on the items on the left menu
https://developer.sabre.com/guides/travel-agency/quickstart