[Boost-users] Boost's Stage\Lib *.a Files

I am running a tutorial program and I found out that in the program's build options under Linker settings/Link Libraries I can put both or either of the ff and the program compiles & runs without errors. What is the difference between the two? libboost_filesystem-mgw8-mt-d-x64-1_77.dll.a OR libboost_filesystem-mgw8-mt-x64-1_77.dll.a