If you see a Calckey user sharing a link to another post with the text "RE: link" that's a quote tweet on Calckey. It "degrades" quite nicely IMO on Mastodon.
Here's an example by @atomicpoet
@donncha Yeah, it's really nice how well it integrates with non-*key services.
Hopefully with the native Quote feature coming to Mastodon soon , we will be able to see those quotes in the same way as they can.