If a proprietary characteristic requires a UUID, a member may generate their own 128-bit UUID. 16-bit UUIDs cannot be used to generate characteristic UUIDs. 

16-bit UUIDs are only to be used to represent services. Please see 16 BIT UUIDs for instructions on requesting a custom service. 

https://www.bluetooth.com/specifications/assigned-numbers/16-bit-uuids-for-members

KB0010305