Theme
Groups and entities
A group is an account or team an event belongs to — the unit you want to analyse by when the customer is a company rather than a person.
An entity is a versioned, reusable object: an account, a project, a workspace. Each entity carries a schema reference, so a consumer reading it knows exactly which shape it is looking at rather than inferring it from the keys that happen to be present. That reference is what makes an entity survive a schema change without breaking every downstream query.
Per-SDK reference
Each page's §8 covers what an entity is, why the schema reference matters, and how it surfaces in reporting.
React: see Browser §8.