OK, I think I got the point now. After relations are created, VFP still does nothing at all to prevent entries breaking the foreign key rule. If I want a real foreign key, I must run the Referential Integrity Builder on my DB. Of course with the drawback that if I change something to the DB and forget to run the builder, I can create a disaster.
And if I read your article correctly, the other annoying thing is that the referential integrity builder will always disallow to set a field to null in the child table, even if the field is defined as nullable ; only the external TaxRI builder would allow this. Right ?