"Global leaderboards" refers to a ranking system that shows the top performers or winners from all around the world in a particular activity or competition. Full definition
There's also a score attack mode that allows you to replay certain arena combat encounters from the campaign in order to compete on global leaderboards. (pcgamer.com)
The menus are laid out slightly differently, but you can still set up squads, look at global leaderboards for challenges and versus levels, and view your friends list. (eurogamer.net)
The game is single player only, featuring «waves» and «endless» game modes with global leaderboards options for each. (appspy.com)