Where's documentation for making Controls?

The Documentation doesn’t give any guidance on writing Controls. And the two examples (“Eye Control” and “HostedPluginExample”) are quite sparsely documented as well.

For instance, what’s the meaning of the two toolbar icon fields? One source says “This is the BMP name” - but they’re declared as longs. Am I supposed to cast those to pointers?
The “flags” are missing some docs as well.
And probably more.

Was there never a doc for controls or did it go missing?

Thomas are there any improvements on this? I am also looking for more information on this. I would like to create ready to use Xojo Controls and haven’t found any other option than using external saved controls/classes.

There never was good documentation. The IDs refer to resource IDs, but nowadays they refer to bmp image files in the plugin archive with matching names.
And for Xojo there is not yet a known way to put high res icons there.