Hi @Giuseppe B.,
you'll find all the details about the OOTB metrics straight from DCRUM. If you create a DMI blank report, you'll notice an Help link on the right side, as shown below:
Click on it and you'll get the whole list of Dimension and Metrics used in DCRUM with the related descriptions, as below:
Hope that helps!
We're working on it right now - the new Getting Started for DC RUM will include information on what's available in a fresh, default installation with Autodiscovery on. It's planned for the next major release, but we may port some of it to the 12.4 doc when it's ready.