Antenna Basics - Directivity formula

Hi,
I've just started studying about Antennas and I'm not able to understand why we need to take phi and theta as pi and 2*pi or vice versa in directivity formula. Could anyone please explain this ? Thanks in advance.

theta will be 2*pi indicating 360 degree rotation on 1 plane. This is required not only to find the gain / directivity on the forward direction of the antenna but also to find the FBR in case of a patch antenna.

phi will be pi (180 degrees) indicating the forward direction only as the FBR is already computed in theta which will be the same.

Theta and phi can be interchanged though. If I am wrong, critics are most welcome.