[Longhanks]

Schengen is NOT a EU achievement.

Nations can sign Schengen, but are never forced to join the EU, nations can be EU members but are allowed to refuse the Schengen treaties.

[tpm]

Schengen is absolutely an EU achievement, although for some time it developed separately; it is now absorbed into EU law. Note that every Schengen country outside of the EU is a member of EFTA and to be in Schengen has to sign an association treaty with the EU.

[eru]

Yes. Though it's part of the same broader 'European project', so it's permissible for people to be a bit lax when informally discussing these things.

[pjmlp]

Where have I said that on my comment?