Hi Olga, CPlanes are used if you want to relocate the grid to a different orientation than its default world axis orientation.

Say for example you have this model and you wish to draw some new details on the highlighted face:



Notice that face is not aligned with the world axes, so if you were to draw in the regular Top, Front, or Right views it would not work easily to align with that particular face since you would be looking at it in a slanted way instead of looking straight down on it.

You can use a CPlane to help with this - the cplane can orient the grid to match that plane and it can also optionally change the Top, Front, and Right views to be looking straight down and from the sides of that face rather than the world axes directions.

So for this example, using View / CPlane let you place the origin point of the new coordinate system, if you snap it on to the center point of that plane it will align with that plane, like this:



Then after pushing "Done" (or using right-click), the cplane will be set to that orientation, notice how the grid in the 3D view is now aligned with that face:



Additionally the Top, Front, and Right views are now looking relative to that face rather than looking in the standard world directions. So instead of having a slanted view of that face, they have a straight-on view of that face.

Now you can draw (in either the 3D view or the "Top" plan view) new shapes and they will be on that plane, like this:




One other note - MoI actually has the ability to automatically align shapes to planes when you draw them in the 3D view, so it is not always necessary to set a custom cplane to draw just one simple aligned shape to an existing plane, for simple things you can just draw directly on the planes in the 3D view and that plane will behave like a temporary cplane when you snap the first drawn point on to it. But if you wish to get your Top/Front/Right views aligned or do more detailed work on a specific plane or set it at a particular angle or something other than the most basic alignment with the plane, then it can be good to set a cplane so the full grid and views get aligned to it.

It tends to be something used with planar shapes, not really as much with free-flowing shapes like flower stuff though...

Also if your shape does align easily to the world axes, it is also generally better to just draw it aligned to those instead. But sometimes there may be just a portion of your shape that does not easily align to the world axes, that's when a cplane can be useful.

Hope this helps explain it!

- Michael