Most of the "Click to:" options are the popular choices from the right-click menu (control-click on Mac), and it's worth trying some out to see when they may come in handy. For example, there's a "Zoom" option that might be just right for some TV screen, or for focusing to take a photo.
One setting that's underutilized is "Sit on object", perhaps because scripted furniture usually also includes touch interaction -- but once seated on a click-to-sit item, an avatar's interaction with the object defaults to Touch anyway. So if the touch menu can wait until you're seated, click-to-sit is perfect -- and of course "Touch" is always available from the right-click option menu even if the object is set click-to-sit.
The most surprising (and most recent) "Click to:" option is "None." One simple use for this might be to mask the interactivity of an object so the mouse pointer doesn't reveal that it's touchable (or sit-able, etc), instead only offering those from the right-click menu.
The feature is really intended, however, to apply to individual links in a linkset, and this is a step in addressing an age-old problem in scripting linked objects: Making just child-prim portions of an object visibly touch-interactive. In the old days, this was done by keeping the touch-interactive bit a child prim and scripting it locally instead of in the root, so only that part had an active touch-related event handler. This led to some awkward linkset designs and extra scripts. (Scripters: There's also llPassTouches() -- which is almost never what you actually want, and won't help keep the root prim from advertising its touchability.)
Even with "None" selected, you can always use that right-click menu to direct a Touch to any link (although that behaviour can be viewer-specific for fully alpha-masked surfaces).
What this setting does NOT do, however, is provide a "click-transparent" click-through surface because "None"-clickable prims still occlude clicks from prims behind them. (All this seems to be counter to reports when the feature first appeared in Firestorm -- hat tip to Juliana Lethdetter. In short, the feature addresses jira BUG-216339 but NOT the related BUG-216375.) Also, there's no way to change this link-specific setting from a script in another link, for now limiting it to stuff you can set once in the object editor.
Reporter Qie Niangao