There is no API access to code-lists classified as non-embedded - external’ e.g. the IASC Humanitarian Clusters which makes it very difficult to get meaningful (for humans) information on activities/transactions coded using these vocabularies.
The proposal is for cases when a non-embedded - external code-list is used, to request/require the user to add the @narrative element (as is currently done for 98/99 vocabularies) to give a some text reflecting the code.
The alternative is that all data users/systems maintain their own copies of all non-embedded - external code-lists, this is unreliable in many contexts.
It is also highly inefficient compared to IATI maintaining these code-lists centrally, i.e. moving commonly used code-lists into the maintained category (and therefore available via the API) which is the other alternative.
See discussion here and the guidance on sector @narrative here.