While implementing HBCI protocol in OpenBanking API gateway I've stumbled upon the fact that HBCI/FinTs protocol allows calling 'custom' bank functions that exist only within one bank.
For example, there is a section Geschäftsvorfälle für den Deutschen Sparkassen-und Giroverband e.V. in HBCI specification that defines such custom functions. These custom functions provide extensions to the default functionality that is declared by the standard.
This means ASPSP (bank) can define, for example, more detailed transaction data if one requests it using a custom function, etc.
The question is - if there is something similar/planned in Xs2a/NextGenPsd2 or in OpenBanking UK?
To be PSD2 compliant ASPSPs follow Implementation Guidelines Xs2a/NextGenPsd2 or OpenBanking UK, but as far as I know both don't have plans to create a standard for custom functions.