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U - User / member ID (message via slackbot IM) eg. U12345678
D - Direct Message (IM, MPDM(Multi-Party Direct Message)) eg. D12345678
C - Channel (Public) eg. C12345678
G - Group (Private Channel) eg. G12345678
@username format as a recipient (-r) is deprecated! Slack API support was removed on 2018-09-12.
Replace it with direct message channel ID or member ID.
https://api.slack.com/custom-integrations/incoming-webhooks#channel_override
We are phasing out support for ambiguously passing a "username" as a channel value. Please always use channel-like IDs instead.
(source: https://api.slack.com/methods/chat.postMessage#channels)
Just one more thing: channels
Consider a future where mentioning channels required using only its ID and channel names were not necessarily unique or following the same structure as #channel names today. That future will one day arrive. Get ahead of the curve and use channel IDs for all indexing and storage organization.
(source: https://api.slack.com/changelog/2017-09-the-one-about-usernames)