Complex sentence: These AVX instructions are in addition to the ones that are 256-bit extensions of the legacy 128-bit SSE instructions; most are usable on both 128-bit and 256-bit operands.
Split into simpler sentences: These AVX instructions are in addition to the ones that are 256-bit extensions of the legacy 128-bit SSE instructions. Most are useable on both 128-bit and 256-bit operands.