From Guild of Writers Wiki

List Of Sequence Prefixes

The Sequence Prefix is a number that will be in the Book that you create in Blender for your Age.

Since the introduction of the GoW Pyprp plugin, sequence prefixes are now obtained using this method: Obtaining your own unique Sequence Prefixes and there is no longer any need to keep a list of these numbers (as it is done automatically by the ULM website). Sequence prefixes issued by the ULM website are unique to you and cannot be given to anyone else.

This is a unique number, and the default number allocated to your Book file when you create your age will be 100.

You must change this number before you export you Age, and try to link in using Uru. Failure to do so will result in the crashing of Uru.

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Reference Informaton Only:

You can find a list of Alcugs Sequence Prefixes here at Alcugs Sequence Prefixes.

If you sign up at the Alcugs Wiki (which is no longer needed if you are using the GoW PyPrp plugin), you can also edit that page and list your own sequence prefix, so that others can see it and know not to use YOUR number.