Units
Units are the unique identifiers for the visitors, they are used to track those visitors over time and across your applications. Units are also referred to as tracking identifiers.
For experiments running across different platforms (iOS, Android, web, email, etc.) the unit should be known across all the platforms. Most likely the authenticated user's user-id.
For experiments running in apps or parts of the website where the user is not authenticated, you might want to create a unit by device. anonymous-id or device-id are common names for those.
For experiments running in email newsletters, you could even have a unit based on the user's email address, but if you are interested in collecting metrics tracked on the website that the newsletter is linking to, it's better to use the same unit that is being used for the experiments running on the website.
Have a look at the Tracking Unit section of the Creating an Experiment docs for more information.
Not all units are equal, some, like user_id, allow for tracking visitors across sessions and even platforms.
Others like anonymous_id are much more transient. This means that the same physical user could potentially have several anonymous_id, (and as a result see a different variant of your experiment).
This is the case for example when they use several devices or if they clear their cookies between different sessions.
While an anonymous_id identifier is likely always available, the user_id might only be available in parts of the applications where the visitor is logged in.