What determines when Visual Studio's Form Designer will allow visual editing?

I'm kind of amazed that I haven't been able to get to the bottom of this. I probably just don't know the right terms to use in my searches:

Some Controls, when dragged from the Toolbox to a From in the VS Form Designer, will show up on the Form more-or-less as it will appear when the program is run. Others, though, will show up as an icon in a tray at the bottom of the Form Designer.

Now, some of those latter are Controls, like Timers and DataSources, that don't actually look like anything, so it makes sense, but I just subclassed the ComponentOne FlexGrid and my subclass won't show up visually, even though C1FlexGrid will.

How does the Form Designer recognize which Controls to display on the Form and which in the tray? What do I need to do to my subclass to get it to show up?

Thanks.