Channel Grouping
Intermediate PronunciationEN/ˈtʃænəl ˈɡruːpɪŋ/
Channel grouping is the rule set that maps every source and medium pair into a small number of named channels shared across reports.
The rules form an ordered list and the first match wins; any pair that matches nothing falls into the unassigned bucket. A growing unassigned share almost always means broken tagging rather than the appearance of a new channel. What causes trouble in monthly reporting is that changing the rules rewrites history in some tools and applies only to future data in others. In the second case a channel's trend breaks exactly on the day the rules changed, and that break is easily mistaken for a real event.
FAQ
It means traffic arrived that no rule recognises, usually from hand-written medium values, typos or untagged links. Before any analysis, open those unassigned rows and inspect their sources, because while that share is large every other channel's percentage is wrong too.