David Cole writes about how Quora defines the responsibilities of Product Designers.

Example from their internal documentation:

Mechanics are the concepts and rules of a product as a user understands them. Namely:
The objects in the system, their attributes, and their relationships to other objects.
Topics are applied to questions which have answers.
Tweets show up in the timelines of the followers of the author.
The actions people can take, under which conditions, and their outcomes.
When a user follows a topic, content from that topic will appear in their feed.
If user A follows user B, user B can message user A, which triggers a push notification for user A.
The information users have access to, under which conditions.
User A can see the topics user B follows on user B’s profile.
A user can see who they have requested answers from, but they cannot see answer requests by other users.

The definition of the role was derived from "adjacent disciplines such as game design, economics, and other social sciences" and not primarily interaction design.