US Server XRay and US Server Charlie both require gold membership to play just like the old EU Spec Ops server did. Maps in these servers are/were mainly infantry only.
US Roadblock and UK War 1 servers are both open to silver members just like the EU Battle Server was. Maps in these servers all have/had some degree of vehicle presence.
I don't know why access is still restricted based on membership now that there are so few players left...it is hard enough to get games going in the NL stat servers as it is