Compiler Compliance Level is What you have selected language level from the available one to set for your project.
While The Build Path is a used to create Java classpath.
In The build classpath it is a collection of paths visible or availabel to the compiler when building the project.
The Configure Build Path dialog is used to manipulate a file in your project called .classpath.Which is normally hidden.
So what the Compliance level you have selected your project will go for that particular compliance level execution.