The records are in a database, there is a serial primary key, you only load messages that have appear after your last downloaded.
If you are using the toolkit for more than just immersion, you will be in a flotilla - otherwise Uboat numbers cannot be enforced (although they could be massaged in-database before issuance).
Short-form messages:
http://users.telenet.be/d.rijmenants...urzsignale.htm