Hi!
Using closed airports as containers for aerial imagery (where secondary LOD20 imagery is not an option due to its huge size) I noticed the following bug with identifiers:
When using 6 digits (XXXX50) as airport identifier:
You see that the information here is a bit contradicting. :)
Obviously the SDK will only allow 5 digit identifiers but it certainly can't hurt to have more digits at your disposal here (maybe up to 256 even?)