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What exactly are IMP bits in ARM (VMSA) paging structures? Documenation says that they mean 'Implementation defined'. But what does that mean? Does it mean they are free to use by software (like AVL bit in x86) or are they reserved for something by hardware and shouldn't be touched?

It means those bits are used by the CPU, but each CPU could use them for something different. You shouldn't touch them unless you know what your CPU does with them.