Monitor Tab in UI

Thanks to help from the FS++ team I have just dicovered the ability to set V Speeds by command and in the UI.  But a couple of questions about some of the other items in the Monitor Tab of the UI under the heading of "Takeoff from: XXXX":

1. What is "Mass (t) and do I need to do anything with it?  Assume (dangerous word) it's for the GTOW in Tons...but is it short Tons (US) or Long Tonnes (UK) since it would need to be calculated from pounds.
2. Is it important that I set the proper takeoff runway in the "Runway" heading...haven't noticed any issues when I don't
3. Do I need to set the actual flap setting I will use for takeoff in the "Flaps" heading...again haven't noticed any issues.
4. What is "Flex Temp"...do I need to do anything with it?
5. Assume (that dangerous word again) that "Tran Alt" is Transition Altitude...what does that mean (transition to what) and do I need to set it before takeoff and what would it be.

Can't find in the manual any explanation for these.

Thanks for any explanations you can provide.