What is first join and best join in xbee s2c?

What is first join and best join in xbee s2c?
DO (Device options)'s bit 2 is 0/1 = First or Best join.
You says that First join means the device joins the network through the first acceptable Beacon response it receives. Best join means the device joins the network through the strongest Beacon response it receives after searching all search mask
channels.
If I select best join of end device’s profile. does it select the best RSSI during nearby routers and a coordinator?
In my test, it’s not…

First join means for the router or end device to Join the first node or network that is allowing joining. Best means that it is to Join the parent node that has the best connection during a Join request.

Then, If I select best join of end device’s profile. does it select the best RSSI during nearby routers and a coordinator?
Is that right? But in my test, it is not.

It will choose the best possible connection (LQI) of the routers and Coordinator that are allowing joining.