can someone explain what

anonymous_id

is defined as?

It depends a little on your application, but it's typically used to experiment on users who are not logged in. So session id, device id, fingerprint, cookie id, or similar.

ahh okay got it

yes, that would work

if we bin at sesssion_id, is there a way we can use user_id

There's no way to 100% stop people from seeing both variations. They could clear their cookies, switch to a phone, etc. We don't do this yet, but we plan to add some data quality checks for this. We can alert on the number of users who saw multiple variations and remove them from the analysis.