Where does the plane exist in this parallel axis theorem?

[Benjamin_harsh]

Homework Statement

MOI of an area w.r.t any axis in its plane is equal to the MOI of the
area w.r.t a parallel centroidal axis plus the product of
area and square of the distance between the two axes.

Relevant Equations

$I_{AB} = I_{GXX} + A.(y^{2})$

$I_{AB} = I_{GXX} + A.(y^{2})$

Same applies to CD;
$I_{CD} = I_{GYY} + A.(x^{2})$

In the above statement, "any axis in its plane" where does the plane exist in this sketch?

 

[NascentOxygen]

The area is that unlabelled rectangular figure, and it's in the plane of the page.

 

[Benjamin_harsh]

The area is that unlabelled rectangular figure, and it's in the plane of the page.

Is that area perpendicular to the plane or parallel to the plane?

 

[NascentOxygen]

Is that area perpendicular to the plane or parallel to the plane?

The area is within the plane. It is that portion of the plane enclosed by four lines.