First year Applied maths question on Inertia Perpendicular Axes Theorem

Hi everyone, hope this is the right forum to put this question.

I've just read the theorem in my study guide called: Perpendicular Axes Theorem

It says that if X, Y and Z are mutually perpendicular axes of rotation, and assuming a rigid body forms a lamina which likes on the XY-plane then

Iz = Ix + Iy (I being moments of inertia about each respective axes)

It looks so simple BUT not every situation is it clear what axes is X, Y, or Z, so say you have 2 out of 3, how do you know that those 2 are the Z and X axes and you need to then subtract rather than add values. Is it always going to be obvious? Is there a way to know?

Thanks!