Purpose and usage of INITIALIZE_PASS_*

Hi All,
Looking at the code of MemDepPrinter.cpp, I saw the

char MemDepPrinter::ID = 0;
INITIALIZE_PASS_BEGIN(MemDepPrinter, “print-memdeps”,
“Print MemDeps of function”, false, true)
INITIALIZE_PASS_DEPENDENCY(MemoryDependenceAnalysis)
INITIALIZE_PASS_END(MemDepPrinter, “print-memdeps”,
“Print MemDeps of function”, false, true)

What's the purpose of the INITIALIZE_PASS_* macros? What's the difference between using static RegisterPass...?
When I try to use these macros in user-defined passes. The compiler complains,

error: ‘initializeXXXPass’ was not declared in this scope
error: ‘void llvm::initializeXXXPass(llvm::PassRegistry&)’ should have been declared inside ‘llvm’

Is it because they should only be used by internal LLVM passes?
I couldn’t find related hints from the documentation.

Thanks!
ryan