Questions are best put in the Help forum.

As for the @ I suspect people use it for locations or contexts. Locations weren't around when RTM started and some people use the tag area for this also.

@ is a useful shorcut/prefix for flagging something as a "context" which basically means either a location or tools that you need to do a task. So @call, @computer, @work, @home @out, etc. It makes a little more sense with locations since you say @ as "at," but it became shorthand for context, which is a Getting Things Done (Book by David Allen about task/project management) moniker.

Also, it helps when alphabetically sorting (which is usually how computers do it), because @computer, @work, etc. will always be next to each other.

Thank you