Data provider internal function reference

I am writing some internal company software to do custom processing on the SQL generated in Web Intelligence. One thing it needs to do is process a couple internal functions, especially @dpvalue. What is the meaning of the function arguments? I know that the second one appears to be a dependency reference to another data provider's field, and the third argument is an optional aggregation function, but what is the meaning of the first argument's string values? I have seen 'A', 'N', and 'D' used.

Thanks in advance.