Question about facebook chat on mobile?

[Rachel L]

When I am chatting with someone on facebook their status changes between chat and mobile status. The chat status has a little green dot to the right of their name and mobile status has a faded icon of a mobile phone. Why is it that their status changes multiple times during the same conversation when it can simply stay the same. I am not familiar with facebook on a mobile phone because I use a computer. Can someone help me understand this? thanks.
I also want to add that when I look at the list of people on the lower right corner of my computer screen there are some that show are on chat with the green dot and there are others that show as on mobile with no green dot. Some of the ones on mobile have for example 10m next to the mobile icon. What does the 10m mean?