Your question is Choosing Randomization Unit for UI Test. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
You are designing a customer-facing experiment for a workflow change in a professional software product. The feature can affect repeated usage across visits, devices, and collaborators, so you need to decide whether to randomize by user, session, device, account, or another unit.
How would you choose the unit of randomization for this experiment? Walk through the trade-offs you would consider, how the choice affects measurement and validity, and how you would validate that the final design is trustworthy.