什么是Channel？
A grouping of traffic sources into buckets like organic search, paid search, social, email, referral, and direct for high-level reporting.
A channel is a high-level bucket that groups related traffic sources: organic search, paid search, social, email, referral, and direct are the standard set. Instead of scanning hundreds of individual referrers, you get six or eight rows that answer "where does our traffic come from" at a glance.
Channels are defined by classification rules: a visit from google.com with no ad click ID is organic search, a visit with utm_medium=email is email, and so on. The rules are only as good as your tagging. Untagged newsletter links land in direct, and a misspelled utm_medium can invent a channel of its own. The other classic mistake is comparing channels on volume alone; email may be a tenth of social's traffic and twice its conversions, so always pair channel volume with a quality metric.
Channel-level reporting is the starting point. When a channel moves, drill into its sources and campaigns to find out why.