There is an extensive list of unit codes for each technology within the respective specifications, however, there are less frequently implemented engineering units being added to field instruments.
For HART Protocol devices, unit codes 240-249 are available for the manufacturer to create their own unit codes. The general rule for unit codes is that if it is something that other people need (industry standard) then submit a request to add it to Common Tables. If the unit code is unique to your company, then feel free to use the 240-249 unit code range with the Classification set to an appropriate value from Common Tables, 5.21 Table 21. Device Variable Classification Codes. See more about reserved unit codes here.
For FOUNDATION Fieldbus devices, unit codes can be requested via Action Request. Once approved, the code will be added to the next release of Tech Note 16 (TN-016).