guidance on transferring secret keys when some of the secret key material is missing
In some cases, people assemble a Transferable Secret Key with some of the subkeys (or the primary key) stripped out.
For example, a subkey might only be on a smartcard, or maybe the keyholder is only authorized to use a subkey, not the primary.
The current specification doesn't explain how to form such an object. Current implementation practice is to use an "experimental" codepoint for the S2K algorithm (aka "GNU Dummy") and just leave out the secret key material.
This practice is widespread enough that we might want to formalize it in the specification so that implementations can interoperate.