Shard Key, Colocated Shard Key [v5]
A shard key is a column in your dataset that represents the actors that performed the actions in your event data. This is the technical implementation that allows Scuba to determine your actors. An actor can be a user, device, or other action-performing entity that you'll use to construct queries.
Colocated shard keys are an alternative to creating new dataset copies when adding shard keys. When you want to establish a new shard key, you usually need to create a new dataset copy. Because each new dataset copy creates a complete copy of the underlying dataset, this linearly increases the storage required in your cluster. Colocated columns allow you to avoid creating new dataset copies when adding shard keys, if the shard key fulfills certain criteria.
If the shard key you want to add has a one-to-one or many-to-one relationship with an existing shard key, you can create a colocated column instead of creating a new copy of your dataset. The important point is that each instance of the new shard key must map to exactly one instance of an existing shard key.