在SLB地层倾角文献中提到Focusing Plane，请问怎么去理解，下面是部分文献。
Focusing Plane Parameters...
The focusing plane and the search angle together serve to limit the search for dips to the directions within the search angle from the direction of the focusing plane.
Focus Plane Type
Three different types of focusing are provided, which can be selected on a zone-by-zone basis in this panel:
•Fixed Plane - the magnitude and azimuth of the plane, and a search angle are required.
•Variable Plane - a dip collections (e.g. output of BorDip), a minimum quality, a standby fixed plane magnitude and azimuth, and a search angle are required.
•Plane Normal to Tool Axis (California option) - only a search angle is required.
