"Subproject of" and "Parent section" conflict
Section hierarchy is build “on top” of the project hierarchy, i.e., first the sections go, then projects. A project define sections for its subprojects – all they are forced to use the section of the parent project.
The plugin does a lot to handle this. Thus, you just won’t be able to set the project section for a child project! But...
When you create a new project the creation form allows you to choose the parent project and the project section at the same time... The latter, actually, will be just ignored, but.