Let us assume we have a rectangular, unswept wing with no dihedral and a given wing twist and we want to measure in flight or calculate its $c_L$ vs $\alpha$ (angle of attack) diagram. Of course each airfoil along the span will have a different local angle of attack at every flight attitude. Which y-coordinate along the span must (or should?) be used as a reference?
In short: it’s up to the designer to pick any definition as long as the design and analysis always obeys that definition.
Usually, it’s a general practice to define a root chord and then calculate the rest of the local aoa‘s in relation to the root chord. This makes wing-strip calculations simpler. It’s also used in helicopter rotor blades and propellers.
On the other hand, there are numerous simplified calculations (that calculate wing or aircraft level properties) that may consider the mean aerodynamic chord. In this case, of course, the mac location would be used.