iTaste VF - where does the o-ring go?

[ninjasinc]

Apologies for the dumb question, but I just purchased an iTaste VF as my first rda, and I'm not really clear on where the o-ring goes. Can anyone shed some light on this for me?

 

[gpjoe]

Is it a spare? Most RDAs/RBAs come with spare o-rings.

Beyond that, pictures or a better description may help.