Caros,
Vendo o documentos de What is New eu não havia percebido que agora os relations serão definidos apenas nas tabelas, para quem assim como eu, deixou passar, basta dar uma olhada na página 51, onde diz:

Q: What you cand do?
A: Migrate Extended Data Type (EDT) relations.
In Microsoft Dynamics AX 2009 the feature was not available
In Microsoft Dynamics AX 2012 use the EDT Relation Migration tool to move the previous relations that are defined under the EDT nodes in the AOT to the table nodes.
Q: Why is this important?
A: In earlier versions of Microsoft Dynamics AX, the relations of tables to extended data types were stored in two locations: relations in tables, and relations in extended data types. In the current version of Microsoft Dynamics AX, relations in extended data types are obsolete. Instead, we recommend that developers migrate their EDT relations to table relations. The EDT Relation Migration tool makes it easy to transfer relations from extended data types to tables.

Percebi isto quando estava lendo o material New Changed and Deprecated Features for Microsoft Dynamics AX 2012

página 8.

Quem quiser saber mais, tem uma ferramenta de migração que pode ser vista aqui: EDT Relation Migration Tool [AX 2012]

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Pichler