Marco Russo and Alberto Ferrari talk about type systems:
In a previous article, Introducing user-defined functions in DAX, we described the syntax for creating user-defined functions, including the two passing modes (VAL and EXPR) and the fundamental parameter types SCALAR and TABLE. In this article, we build on that foundation and focus on the complete type system, with particular attention to the reference types introduced in March 2026 that provide better documentation, stronger validation, and improved IntelliSense support.
Before diving into the new types, let us briefly recap the full picture of parameter types and passing modes available in DAX user-defined functions.
Click through for a classic deep dive from Marco and Alberto.