From the book "Custom Shape Tutorial" by Regina Henschel[1]:

> [Custom shapes] are written
> as element <draw:custom-shape> in the document source file. LibreOffice provides
> a lot of predefined custom shapes. They are grouped to the sets ‘Basic Shapes’,
> ‘Block Arrows’, ‘Symbol Shapes’, ‘Stars and Banners’, ‘Callouts’, and
> ‘Flowchart’. And all shapes from the ‘Fontwork Gallery’ are custom shapes too.
> To distinguish a custom shape from other kind of shapes select the shape and
> look at the tool ‘Toggle Extrusion’ . It is only enabled, if the shape is a
> custom shape.

[1]: https://documentation.libreoffice.org/assets/Uploads/Documentation/en/Tutorials/CustomShapes7/Custom-Shape-Tutorial.pdf