jdbc/Main is indeed the main (default) data source.
All database-related plug ins access this data source by default. In order to access any other data source, you will need to specify it explicitly using the optional <Data Source> trigger. It therefore makes sense to name the database to which the model refers most often as jdbc/Main to avoid having to specify it again and again.
I agree that documentation can be improved in this respect (as indeed in many others), and we are making an effort to put more emphasis on this aspect of our product.
Thanks for your support.
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