What is the true meaning of halfApertureAlongTrack and halfApertureAcrossgTrack

Hello, I would like to ask how to understand the half Aperture AlongTrack and half Aperture AcrossTrack here. I would like to know what exactly these two angles correspond to in the actual satellite sensor?

// Defining rectangular field of view
double halfApertureAlongTrack = FastMath.toRadians(20);
double halfApertureAcrossTrack = FastMath.toRadians(20);
FieldOfView fov = new DoubleDihedraFieldOfView(Vector3D.MINUS_I, // From satellite to body center
Vector3D.PLUS_K, halfApertureAcrossTrack, // Across track direction
Vector3D.PLUS_J, halfApertureAlongTrack, // Along track direction
0); // Angular margin

    // Defining attitude provider
    AttitudeProvider attitudeProvider = new LofOffset(inertialFrame, LOFType.LVLH);

    // Defining your propagator and setting up the attitude provider
    Propagator propagator = new KeplerianPropagator(initialOrbit);

This is the source code from the orekit library´╝Ü
public DoubleDihedraFieldOfView(final Vector3D center,
final Vector3D axis1, final double halfAperture1,
final Vector3D axis2, final double halfAperture2,
final double margin) {
super(createPolygon(center, axis1, halfAperture1, axis2, halfAperture2), margin);