When constructing AIRIMP messages such as PNLs, I have a few questions about
the flight element, which my IATA manuals doesn't seem to answer...

The flight element is constructed like this:
Flight# / Date [space] BoardingAirport [space] Part#

Example:
IB881/05MAR AMS PART1

First question: If the flight is a multi-legged flight, does the
BoardingAirport change on the flight element for each PNL sent to the
different airports on the route, or does it always indicate the first
airport on the route?

Second question: What does "part" mean? Is it for type-B messages that has
been splitted up, or which leg of the flight the PNL applies, or something
else?

Thanks in advance!

Regards,
Martin