HPB, for Heading, Pitch and Bank, refers to the rotational coordinate system used within Cinema 4D. These coordinates tell Cinema 4D how you wish the object to rotate about its axis (or axis offset). Heading is the direction axis. This axis tells you where your object is pointing, or following its nose. Pitch is the up/down axis of rotation, like nodding your head yes. Bank is the rotate along the center (heading) axis. The bank axis is relative to your object. The heading and pitch axes are not, and instead are relative to either the coordinates of the parent (if there is one) or the world coordinates.