Hey mikihirir,
i think we got the same problems on our visuals 
.
I do not found this in the API but i think you are right. In the moment we only have that three options:
Category, Measure, Series.
Maybe you can use the Series in a Table for the selectionid. So every row is one id.
But i do not try it out know.
In my case i used one category. Because when you define a table in the capabilites. The tool creates everytime a categorical.
The fields do not have so much metadata. But you can identify the categorical data by id and the column id in the table.
My workaround is very complicated and i think we can make a better solution for this.
Nice Regards
Mcburn