Can someone maybe help me understand how to have specific retention policies for different departments / countries.
If I select specific locations and choose recipients there is a message saying that:
Distribution lists and mail-enabled security groups will be expanded so the policy will apply to all the mailboxes in the group. Only the mailboxes that are currently members of these groups will be covered by the policy. Mailboxes added to these groups later won't be covered, but ones that are removed later will still be covered.
I can't find this information anywhere else, the overview only mentions a limit on mailboxes and sites.
How can we apply a retention policy to a unique group and make sure that new mailboxes get the policy? Or is it better to use automatic labels?
You simply need to make sure to add any newly provisioned users into the policy scope, as mentioned in the text you pasted above group membership will not be re-evaluated once the policy is created. PowerShell can be used to automate this, if needed.