This option allows you to connect and play with/against other players over the internet. There are four options to select once you select this option.
Connect with your friends and battle it out in Smash and Team Smash modes. For the first match, the host will be able to set the match rules. In the succeeding match, the player on last place will be able to modify the Rules for the next match. This is also the only mode that allows the usage of customized fighters.
This is the actual online multiplayer mode where you can play against players around the world. There are two options: Play for Fun or Play for Glory. The first one are for casual, non-competitive players. Battle stats are not recorded, and all items, and stages (except Final Destination) is enabled. The latter is for the competitive players. Wins/ losses are recorded, items are not enabled, and battles are confined in Final Destination Forms of stages. In addition to the default Smash and Team Smash, players can also go head-to-head against a rival in the option's 1-on-1 selection.
This mode will allow you to participate in special Conquest events and support your preferred team. When a Conquest event is active, you can play as one of the featured characters in play "With Anyone" to earn points for that character's team. The team with the highest total points at the end of the event wins.
If you don't feel like fighting or you just want to give yourself a rest, spectating other players' replays may give you research material and ideas about tactics and character moves/combos you're not familiar about. Aside from that, you can also bet gold on online battles or review the worldwide stats.