Originally Posted by KVS
As a general rule, US-based carriers do not allow their own elites access to lounges when travelling on "domestic" (non-international) itineraries (their definition of domestic/non-international generally means flights within the US, between US and Canada, and between US and some Caribbean countries).
This restriction only applies to their own members (e.g. it would apply to DL SkyMiles & CO OnePass members, but it would not apply to OK Plus or AF F+ members).
The same rule is also applicable for the *A and OneWorld lounge access...
This is not correct. The alliances have different rules. What you describe is true for *A and OW for not ST. The official ST policy is not to grant access unless you have an international ticket for the same day. Now, YMMV and some agents will let you in if you have a non US card, but this is not official policy.