Ultimately if submarine designers et-al have enough unregulated address space for services and subjects to work with then I don’t think we have much to argue about here. I agree with @kjetilkjeka that forcing DSDL to always error when using regulated IDs from custom types is draconian and not true to the open-source spirit. I think a default failure that can be demoted to a warning with a switch is a good approach (if you mention the switch in the error message, that is). I also agree with @pavel.kirienko that, for example; a certified submarine type would never undergo a complete reallocation of port identifiers just to accommodate COTS hardware but that such systems might incorporate equally certified COTS hardware where it could be purchased as an LRU. This latter point drives the requirement that we error by default and perhaps nag a little when using the override but, coming full circle, the only reason makers and submarine designers wouldn’t just stick to the unregulated port identifier spaces is if these spaces were too small.