Registrations are open until mid-July, approximately.
A booth is defined as an area in which your creation (mod, modpack, server network, OpenComputers program, etc.) will be showcased. If you want to host one, you should be available for at least some of the convention (with voice chat - this is key), have your mod ported to Minecraft 1.9.x by mid-July and be prepared to show off your stuff! (Automated booths which don't require a host are welcome, but we'd still like to see you at the convention itself. More fun that way.)
A panel is defined as a talk in which you (alone or as a group) discuss/showcase/talk about something - anything, really! - in front of an audience. If you want to host one, you should be available for its duration (with voice chat - this is really key this time) and be at least somewhat prepared to prevent the chaos of the previous two conventions. (A way of creating slides for OpenComputers will hopefully be documented soon.)
Both booth and panel hosters will be given early access to the server in order to prepare the booth or to practice/test things for a panel.
If you want to propose something that's not a booth nor a panel (such as a parkour course, a building, some kind of extra stuff), go ahead! I'll listen!
I will start approving booths and panels soon, however do keep in mind that in order to ensure variety and quality, not every idea has to be accepted - I'm generally fairly open, however.
Yes, you can use the form more than once. One person can host multiple panels and/or run a booth for multiple mods.