The definitive reference books for the ORM1 and ORM2 languages are:
ORM1: defined in the book "Information Modeling and Relational Databases" published in 2001 by Morgan Kaufmann
ORM2: defined in the book "Information Modeling and Relational Databases: Second Edition" published in 2008 by Morgan Kaufmann
The ORM tools do not implement all of the ORM features defined in the books.
I believe that the intention is for a future release of NORMA to fully implement ORM2.
ORM1 is used by InfoModeler, VisioModeler and VEA.
To the best of my knowledge, ORM2 is used only by NORMA and ORM Lite.
ORM2 subtype constraints are explained on page 242 of the ORM2 book (see above)
And, as you say, you cannot show subtype constraints in VisioModeler.
Microsoft has archived VisioModeler so it is no longer available for download from the Microsoft website.
Hope this helps.