Here are links to both official apps:
And a picture of all apps in Slack directory. I understand, that I should use EU versions (being located in EU). But not sure what is the difference.
Which apps are you considering?
There are more than one because Atlassian is an open ecosystem - they actively encourage people to write apps that run inside their applications or draw Atlassian stuff into other places, making them more accessible and flexible.
There's more than one Slack app because more than one person/organisation has chosen to write one.
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