How can CSXS Manifest element "UI" be minOccurs="0"

[SamuelCharpentier]

I was testing different manifest.xml settings to better understand it and I came across this issue:

If the DispatchInfoList > Extension > DispatchInfo > UI > Type is anything but 'Custom' (invisible extension), then UI > Geometry > Size is required. Otherwise, the extension is not listed in the Window > Extensions list (tested in InDesign and Illustrator).

I did not try to open a DispatchInfo > UI > Type of 'ModalDialog' with no Size from another extension yet, but I'm doubtful it would work as a DispatchInfo wiht Type 'ModalDialog', Menu 'Name' and Geometry > Size will be in the Window > Extensions list but one without a Geometry > Size wont.

Could anyone give me an example of a working extension where UI is not in the manifest.xml (since that element is minOccurs="0", as displayed in Adobe's manifest.xml example/definition) and still end up with a working extension.

Does it mean that by default the UI > Type is Custom and the extension is invisible by default, therefore can't be listed in the windows extensions?

How does it handle a manifest.xml with no UI?