For example on one of my machines I have max memory set at 26,624 MB, which is my buffer size, the plan cache is an additional 4,408 MB.
The plan cache is part of the buffer cache, so your plan cache takes up 4408MB of the 26624MB buffer pool. What's outside of the buffer pool is things like CLR memory, backup buffers, thread stacks, linked server drivers and a couple other things. They're generally smallish memory allocations
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