After study these documents. (Wow, this document was posted on March 25th, 2015. It is a newest feature!)
I found that "Session" are records which created while user login, and destroyed after user logging out. One login on any device will have a "Session" object, so there will be 3 Session objects if user login on 3 devices. (But I am not sure that session will be destroyed while app had been closed by OS or not.)
User could have a custom "Session manager" which provided by app designer to know how many devices had login this account.
I could see "Session" page after adding a line into my plugin, I will update my auth plugin later.
And the Session object will not be destroyed if closing app without all logging out.