what is reffering this code
<?xml version="1.0" ?> in
<layout version="1.0.0"> in layout
custom_module.xml file ?
<?xml version="1.0" ?> defines the xml document. While there already is a newer 1.1 Version you still will mostly see the 1.0 Version: https://www.ibiblio.org/xml/books/effectivexml/chapters/03.html
the layout tag is a Magento construct and I assume that its Version is 1.0.0 since the beginning of time, but was introduced to have the possibility to upgrade the layout definition over the time.