A null is a custom object, that doesn't render, but is visible in the scene.  In other software packages, nulls are commonly used for camera and spotlight "look at" targets, for example.  A wfmm null is visible in the scene as a small star object, almost a 3D asterisk.

Some people use a simple primitive, such as a cube, painted completely transparent, as a pseudo-null in their scenes.  This does work, but has problems - you have to make sure you disable it from any collision, and it can be inadvertently included in custom scene exporters, etc.  While not major problems, they still require a bit of extra effort to work around them,  impeding workflow.   With cp4's null, you just click.

Examples:

overview

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