What does the Facebook Chat icons mean?

[Allira]

Okay I know it's a random question, but say for example I log on to Facebook, open up the chat box and next to a friends name there is a grayed out mobile symbol what does that mean? Are they on Facebook or are they offline? And if they come online does it turn into a green dot?

Thanks

[Deepak]

Mobile icon means the person is using mobiles to use facebook. (example- android users).
Mobile users' icon does not turn green because they're always online.
Others, if it turns green then they are online and available to chat.