The Linkshells are chatrooms, they're kind of a relic of the olden times, doesn't mean they're no longer used, but as players nowadays have much better unofficial alternatives to them they're not as prominent. The main ones you hear about nowadays are ones setup for very specific purposes like Hunts.


And for duties, the biggest reason for why they are Data Center wide and not World Wide is probably to avoid adding in extra lag to people who'd be connecting overseas. Kind of why at first Data Center travel when it gets implemented in 6.x, it'll at least at start only be limited by region (EU, NA or JP).