flowing-retail | Sample application demonstrating an order fulfillment

[Let me first clarify the expression "picking the compiler version" you use in your question. The compiler you use is configured through the compilerId and compilerVersion options and it's usually the compiler of the JDK you installed. We are talking about setting the compatibility options of your compiler.]

The properties you mention are used to provide the -source, -target and --release arguments to the javac compiler (or equivalent if you use another compiler).

Since --release was introduced in Java 9, which one applies depends on the JDK used to run maven. If you provide the --release argument:

• on JDK 9 and higher, maven will ignore the -source and -target parameters,
• on JDK 8 and lower, the compilation will fail miserably.

The maven-compiler-plugin specifies three ways to provide these parameters:

1. in the section of a tag. This one overrides the others,
2. as a property in your pom.xml. This one applies if you didn't specify the parameter in a section,
3. if you didn't specify any of the above, a default for -source and -target applies (--release does not have a default).

You can find the defaults in the maven-compiler-plugin.jar (resource META-INF/maven/plugin.xml):