Question

what information should be included in formal written confirmations from users about the new systems?

Answer 1

A user story is an informal, natural language description of one or more features of a software system. User stories are often written from the perspective of an end user or user of a system. They are often recorded on index cards, on Post-it-notes, or in project management software. Depending on the project, user stories may be written by various stakeholders including clients, users, managers or development team members.
Reference:
https://en.m.wikipedia.org/wiki/User_story