desruisseaux commented on code in PR #1936:
URL: https://github.com/apache/maven/pull/1936#discussion_r1857010986


##########
api/maven-api-model/src/main/mdo/maven.mdo:
##########
@@ -1945,6 +1965,178 @@
       </codeSegments>
     </class>
     <class>
+      <name>Source</name>
+      <description>
+        <![CDATA[
+        Description of the sources associated with a project main code or unit 
tests.
+        The sources can be Java source files, generated source files, scripts 
or resources for examples.
+        A source is specified by a mandatory {@code directory} element, which 
is relative to the POM.
+        The directory content can optionally by reduced to a subset with the 
{@code includes} and
+        {@code excludes} elements. The kind of sources (codes, resources, 
<i>etc.</i>) and their
+        usage (main code, tests, <i>etc.</i>) is specified by the {@code 
scope} element.
+
+        <h2>Default source directories</h2>
+        If no source directories are specified, the defaults are {@code 
src/${scope}/${lang}} where
+        {@code ${scope}} is the value of the {@link #scope} element (typically 
{@code main} or {@code test}) and
+        {@code ${lang}} is the value of the {@link #lang} element (typically 
{@code java} or {@code resources}).
+        ]]>
+      </description>
+      <version>4.1.0+</version>
+      <superClass>FileSet</superClass>
+      <fields>
+        <field>
+          <name>scope</name>
+          <version>4.1.0+</version>
+          <description>
+            <![CDATA[
+            Specifies in which context the source files will be used - 
typically {@code main} or {@code test}.
+
+            <p>The <b>main</b> scope is used for specifying a directory 
containing the source of the project.
+            The generated build system will compile the sources from this 
directory when the project is built.
+            The path given in the {@code directory} field is relative to the 
project descriptor.
+            The default directory for the default language (Java) is {@code 
"src/main/java"}.</p>
+
+            <p>The <b>test</b> scope is used for specifying a directory 
containing the unit test source of the project.
+            The generated build system will compile these directories when the 
project is being tested.
+            The path given in the {@code directory} field is relative to the 
project descriptor.
+            The default directory for the default language (Java) is {@code 
"src/test/java"}.</p>
+
+            <p>If no scope is specified, the default is {@code main}.</p>
+            ]]>
+          </description>
+          <type>String</type>
+          <defaultValue>main</defaultValue>
+        </field>
+        <field>
+          <name>lang</name>
+          <version>4.1.0+</version>
+          <description>
+            <![CDATA[
+            Specifies the language of the source files - typically {@code 
java} or {@code resources}.
+            Resources is used as a generic term for scripting languages (e.g., 
JavaScript or Python)
+            or markup languages (e.g. properties file, <abbr>JSON</abbr> or 
<abbr>XML</abbr>).
+
+            <p>The <b>java</b> language is used for specifying a directory 
containing the Java sources of the project.
+            The generated build system will compile the sources from this 
directory using the Java compiler when the
+            project is built. The path given in the {@code directory} field is 
relative to the project descriptor.
+            The default directory for the main Java sources is {@code 
"src/main/java"}.</p>
+
+            <p>The <b>resources</b> language is used for specifying a 
directory containing the class-path
+            or module-path resources such as properties files or scripts 
associated with a project.
+            This directory is meant to be different from the main source 
directory,
+            in that its contents will be copied to the output directory in 
most cases
+            (since scripts are interpreted rather than compiled).
+            The default directory for the main resources is {@code 
"src/main/resources"}.</p>
+
+            <p>If no language is specified, the default is {@code java}.</p>
+            ]]>
+          </description>
+          <type>String</type>
+          <defaultValue>main</defaultValue>
+        </field>
+        <field>
+          <name>module</name>
+          <version>4.1.0+</version>
+          <description>
+            <![CDATA[
+            Name of the Java module (or other language-specific module) which 
is built by the sources.
+            This element can be specified in a Maven project containing 
multiple Java modules.
+            It is generally not needed for non-modular projects, or for 
modular projects having only one module.
+
+            <p>If a module name is specified for resources or script files,
+            then this value modifies the directory where the files will be 
copied.
+            For example, if a Java module name is "foo.biz", then the {@code 
foo/bar.properties}
+            resource file will be copied as {@code 
foo.biz/foo/bar.properties}.</p>

Review Comment:
   It is a Java compiler convention. In a multi-modules project, `javac` 
automatically inserts module names at the root of class directories. For 
example, if a module named `my.foo.module` contains a `foo.bar.biz.Bla` class, 
then in single-module project (as today), `javac` generates the following file 
in the Maven `target/classes/` directory:
   
   * `foo/bar/biz/Bla.class`
   
   But in a multi-modules project (would be optional and not the default), 
`javac` generates the same file at the following location instead:
   
   * `my.foo.module/foo/bar/biz/Bla.class`
   
   Other Java tools such as `java` understand this directory hierarchy. I.e. 
the `--module-path` can be a directory of modules, with no need to enumerate on 
the path each module found in the directory. I'm not sure if it is a 
requirement that the directory names match module names, but this is what 
`javac` does at least by default.
   
   Above documentation of `module` element tries to match what `javac` does. It 
could be rephrased as _"During Java compilation, we let `javac` do whatever it 
wants. For resources, we try to copy them in the same directory as `javac`"_. 
The current wording tries to be more specific about what happens, but the goal 
is to match `javac` and the documentation would be updated if we find 
mismatches.



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