Jitsi, two SIP accounts, and outgoing calls

I could really use a little help. Freshman, installed jitsi, version 2.10.5550, under os-x.
Setting up jitsi to connect to my (first) SIP account worked like a charm. Next I installed a second SIP account using the same procedure. The jitsi “options” window now shows 2 SIP accounts, both online. Here are my questions:
a) what do the check-marks in front of an account (in the options dialog) mean ?
b) what happens if the check mark is set or cleared ?
and most important
c) how do I tell jitsi which account / line to use for the next (or any) outgoing call ?

Thank you all !