Traverse from FBK file

Hello,

I found an example of FBK file but I have a problem with BS definition. I do not know what is ORIENTATION?
BS syntax is BS [point] (orientation) on the Autodesk website, but I could not find any description of it.
can you please explain more about orientation value.
Thanks in advance

STN 2 5.07500000 ""
BS 1 0.00025000
SF 0.99986902
AD 1 0.00025000 NULL "A"
PRISM 4.77000000
F1 VA 1 0.00000000 563.03000000 90.39400000 "A"
F2 VA 1 180.00050000 563.02000000 269.20550000 "A"

I forgot also to ask about NULL value in AD. why NULL is typed instead of a distance?

any help ?