Add documentation of the steps that OVAL content goes through during the build

docs/manual/developer/07_understanding_build_system.md

@@ -86,6 +86,13 @@ of occurrence:
 - Generate content for derived products (such as CentOS and Scientific Linux).
 - Generate HTML tables, Bash scripts, Ansible Playbooks and other secondary artifacts.
 
+## How OVAL is build
+
+Creating OVAL checks is done in two steps. First, all available OVAL checks are combined into a one unlinked OVAL document. The OVAL shorthands are loaded into the OVAL Document object and, in the case of template shorthand, extended using jinja macros before loading. If the shorthand is already loaded into the OVAL Document object, it is skipped. Shorthands are loaded in the order that benchmark checks are loaded first, followed by shared directory checks.
+
+The second step is to link the generated document from the previous step with the XCCDF document.
+The unlinked OVAL document is loaded into the OVAL Document object while the XML file is being loaded into the object instance. Validation of the OVAL document is performed during and after loading (for example, whether all the checks listed in the XCCDF are present in the OVAL document). After validation of the OVAL document, the IDs are converted to valid OVAL IDs. The OVAL document is then saved as an XML file. During saving, a minimal OVAL document is generated for each rule as an artifact. The last step is to link the file to the XCCDF document.
+
 ### Python Build Scripts
 
 Various Python utilities under `/build-scripts` contribute to this process;