Build off là gì

Hello!

I am reading a sustainable book, and I found the following phrase: "There are two additional healthcare credits in this category that build off the idea of providing accessible open space".

What does "build off" mean in this context?

Can somebody help me please?

Why does the author add the "off" to the "build" word?

What happens if the author write "There are two additional healthcare credits in this category that build off the idea of providing accessible open space" instead?

Thank you!

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