In a multi-module build, why does a child module need to be told where to find the parent POM? - maven

Question

Consider the following scenario of a flat multi-module layout:
| parent-pom
| - pom.xml
| module1
| - pom.xml
Where parent-pom/pom.xml is the parent POM of all modules:
<groupId>my-group</groupId>
<artifactId>parent-pom</artifactId>
<version>1.0.0</version>
...
...
<module>../module1</module>
Now, the pom.xml of module1 contains the following parent section:
<parent>
<groupId>my-group</groupId>
<artifactId>parent-pom</artifactId>
<version>1.0.0</version>
</parent>
I'm starting with a clean local repository; none of the artifacts is pre-built, everything done from scratch. Trying to execute mvn install on parent-pom will result in an error, because the Maven reactor will look for my-group:base-pom in the local repository, fail (because it's not there) and then look for ../pom.xml.
Fine. My question is this: if the build of module1 is invoked through the build of parent-pom, why does Maven even have to look for the parent's pom.xml anywhere? when Maven comes to build module1, it already knows the following things:
The physical location, of the file system, of my-group:parent-pom:1.0.0.
The fact that module1 is rooted in my-group:parent-pom:1.0.0.
Why look elsewhere?

Answer 1

The Introduction to the POM:Project Inheritance:Example 2 told us as the following: -
The Scenario
However, that would work if the parent project was already installed in our local repository or was in that specific directory structure (parent pom.xml is one directory higher than that of the module's pom.xml).
But what if the parent is not yet installed and if the directory structure is
.
|-- my-module
| `-- pom.xml
`-- parent
`-- pom.xml
The Solution
To address this directory structure (or any other directory structure), we would have to add the <relativePath> element to our parent section.
<project>
<parent>
<groupId>com.mycompany.app</groupId>
<artifactId>my-app</artifactId>
<version>1</version>
<relativePath>../parent/pom.xml</relativePath>
</parent>
<modelVersion>4.0.0</modelVersion>
<artifactId>my-module</artifactId>
</project>
As the name suggests, it's the relative path from the module's pom.xml to the parent's pom.xml.
EDITED:
The getRelativePath told us as the following
Get the relative path of the parent pom.xml file within the check out. The default value is ../pom.xml. Maven looks for the parent pom first in the reactor of currently building projects, then in this location on the filesystem, then the local repository, and lastly in the remote repo. relativePath allows you to select a different location, for example when your structure is flat, or deeper without an intermediate parent pom. However, the group ID, artifact ID and version are still required, and must match the file in the location given or it will revert to the repository for the POM. This feature is only for enhancing the development in a local checkout of that project.
I hope this may help.