Determines the cardinality / dimension of how requested fields are grouped:
domain: Results will be reported at the domain level
campaign: Individual results by journey
step: Results by step within a journey
channel: Results broken down by the delivery channel (WEB, NATIVE_IOS, NATIVE_ANDROID)
variant: Results by the variants of a split test
split_test: Results by split test
Not every combination is accepted. step, variant and split_test each describe something inside a journey, so they pair with campaign and with each other only as listed: variant and split_test require campaign, and neither combines with step or with channel.
A variant or split_test breakdown also returns revision_id whether or not it was asked for. A variant keeps its id across revisions of a journey, so without the revision two rows describing different edits of the same variant cannot be told apart.