Spring Boot repackages the jar file for this project using a custom layout factory defined in the additional jar file, provided as a dependency to the build plugin:

              <layoutFactory implementation="com.example.CustomLayoutFactory">

The layout factory is provided as an implementation of LayoutFactory (from spring-boot-loader-tools) explicitly specified in the pom. If there is only one custom LayoutFactory on the plugin classpath and it is listed in META-INF/spring.factories then it is unnecessary to explicitly set it in the plugin configuration.

Layout factories are always ignored if an explicit layout is set.