Compiler Compliance Level vs Build JRE

I am working on a eclipse project for which default build project library is 1.5 but compliance level is 1.4.

From above configuration what I understood is that the project will be build with 1.4 compatibility and build the same way as that of JDK 1.4 library.

In my program I am using java.util.concurrent.CopyOnWriteArraySet which was introduced in jdk 1.5. When I compile my code with above setting it gets compiled properly.
But when I point my default workspace library to 1.4 with compliance level as 1.4 it shows error for the same.

What is the difference between setting the build library JRE and compiler compliance

Please reply ...

Harry, welcome to Java Ranch!

I just knew that I recalled seeing this discussed earlier, and sure enough searching for "compiler compliance level" in this forum yielded this:
http://www.coderanch.com/t/531065/vc/Difference-between-Installed-JREs-Compiler