For the Slack Integration - A custom activity would be if someone added an emoji to a message for example. Since I've noticed not that many people are sending messages but instead using emojis. We just want to see who is active and interacting with our slack messages we post.
Merged in a post:
Track Discord Reactions
Some channels in Discord focus on having members reacting instead of messaging, like announcements e.g. Some users don't even chat in server and just react to messages, polls, reaction roles, etc. As a community manager it should bring a lot of value to track this and provide even more granularity in community activity reporting.