This cove and astragal will introduce a side round. It is a very specific plane. If you want to do anything and everything you must have a pair. They allow you to plane up to another facet.
I use this just I like a rabbet plane. Mark a gauge line and carefully start the cut using your fingers as a fence. After a few passes you can zip through it.
This diagram shows the side round knocking off the tip of the highest rabbet. I usually avoid this by laying out a circle that's slightly bigger in diameter. In this case, where the diameter of the circle is equal to that of the side round, I attacked some of those rabbets with the 10 round in step 1a, which is not pictured.
I try to avoid the scraping cuts that happen near 90 degrees. Like a snipes bill, most of the time I only use the tip of the profile.
This is as close to freehand as you'll get. I used a #10 in the pictured profile below
Why is a side round necessary here?
The astragal prevents the #10 from reaching into the corner.
Like snipes bills, side rounds come in pairs because there are times you will want the profile facing the opposite direction.
There are other uses as well. We'll see some of those later.