I'm afraid I don't know what "Holdings Based Classification" is. The rule of the thumb is, if you can retrieve a datapoint using =TR function in Eikon Excel, you can also retrieve it using Eikond Data APIs, and you would use the same field name and parameters.
If you're looking for the field name for a data item, I suggest you use Data Item Browser app in Eikon or Formula Builder wizard in Eikon Excel as described in this tutorial.
If you cannot find what you're looking for it's possible that this data item does not exist in Eikon. To verify this you can contact Thomson Reuters Helpdesk by either calling the Helpdesk number in your country or by using Contact Us capability in your Eikon application.