Differences between "empty project" and "external build system" in XCode

I am new in the programming world, and use XCode's IDE 开发者_开发百科and compiler (GCC?) to program. But I have a questions, what is the differences between "empty project" and "external build system" in XCode?

It's sounds like a stupid question, but why not ask? :)

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The External Build System project template already contains a target for, you guessed it!, an external build system; the empty project is empty, i.e. has no targets at all. Not that big a difference, there is a template for an external target, so it's just a few clicks away.