This iExpression is applied to the orientation of a 3D layer. It rotates the layer such that it always "looks" towards a given line. Each layer looks at the point of the line that is closest to it. In other words, the direction in which the layers look is perpendicular to the direction of the line. The line has an infinite length and is given by two (arbitrary) points on the line. The rotation can still be adjusted using keyframes as usual. You can, for example, align layers on the surface of a cylinder by choosing as line the axis in the middle of the cylinder.
Line Point 1
is the first point on the line at which the layer should "look".
Line Point 2
is the second point on the line at which the layer should "look".
flips the rotation such that the layer does not look at the line but looks away from it (i.e. turns its back on it).
if you choose 'other point' here, the layer looks not from its own position but as if it was located at some other position. If you want, for example, that the layers looks in exactly the same direction as another layer which uses this iExpression, then you can simply link this other layer's position to 'other point'.
is the point at which the layer should "look" when the option 'other point' has been choosen for 'Look From'.