The HART Specifications describe the expected behavior for each command. Anything other than the described standard behavior is considered manufacturer specific behavior and will not qualify for registration with FieldComm Group. 

HART allows device developers to implement custom commands, just be sure to include it in the Device Support Files (EDD/FDI Package).  

Please see Table 9, HART Command Number Partitions, in the Command Summary Specification for the available Device Specific Command number ranges.