A hole is the absence of a key, which has keys below it, e.g. if some/key is missing in a property file:

some = value

some/key/below = value

some has a non-leaf value. Another example of a non-leaf value in XML (abc): <abc>value<def>value2</def></abc> interpreted by xerces plugin:

abc/def = value2

abc = value

Config files ideally do not copy any structure if they only want to set a single key.

strongly hierarchically structured data must still be supported

data structure must always be complete
prohibit non-leaves values

Support holes and values for non-leaves in a KeySet if the underlying format allows it.

If the underlying format does not support it and there is also not an obvious way how to circumvent it – e.g., JSON which does not have comments – holes and values in non-leaves can be supported with key names starting with ®elektra.

It fits very good to the idea of key-value.
Some formats support it (e.g. XML supports non-leaves values; property-files support holes).
It can be useful for migration purposes, e.g. there is /some/key, and later /some/key/enable gets added. Then it is beneficial if /some/key still can hold a value.