Feature ID, should be unique, inmutable identification for physical
Feature being modeled.
Care must be taken to avoid generation based things that will change over
time (mutable attribtues, or shapefile rows). It is hoped that the domain
being modeled will have specific identity practices in palce that you can
depend on, as example Airports make use of standards code such as YVR
for the Victoria International Airport.
As a consequence of this different Application Descriptors representing
the same physical content should end up with the same idenification. This
is a "dream", on the off chance is obtained you may wish to prepend the
typeName to this ID when producing GML. This would only be a problem when
creating one GML document based on two application schemas for Airport,
both of which succeeded in using the actual physical identification
strings for their ID.