What are HUD attachments
In fighter aircraft and first-person shooters, the HUD (or Heads-Up Display) shows the user important information directly in their field of vision. In Second Life, HUD attachment points have been added to the right-click/pie menu and inventory right-click menu for objects you own. To attach an object to your HUD, right-click it, choose 'Attach HUD >' and pick one of the eight attachment points. The object will be attached to your viewer and only you will see it.
The biggest advantage to HUD attachment points is that they do not show up in-world as attached to your avatar. Other Residents will not see them. Many applications for this feature can be imagined, including games where you do not want the other players to see what you have; Informational displays such as speedometers, simulator statistics, radars, ammunition remaining, etc. and photo slideshows of your grandmother's visit to the largest ball of cling-wrap in the world. Subjecting others to that should be abuse-report-worthy. Keep them in your HUD!
HUD attachments can be scripted to take advantage of LSL, with a few limitations. Particles do not work on HUD attachments. In addition, there are a few known issues with HUD attachments. HUD attachments obscure chat bubbles if they overlap. HUD attachments can not be clicked-through because HUD attachments can receive touch events (by design). HUD attachments only appear in snapshots if you have the 'UI in snapshot' option enabled.