Hi @Ontsnapt ,
I did not find useful information about Sybase SQL Anywhere CREATE DOMAIN Conversion to SQL server. Please read below blogs.
Sybase SQL Anywhere CREATE DOMAIN Conversion to Other Databases
SAP Sybase SQL Anywhere to SQL Server Migration
If you want to find information about User-defined SQL Server Types. Suggest you reading the blog An Overview of User-defined SQL Server Types to get more details. It shows the detail examples.