All market makers are HFTs. Most (but not all) HFTs are market makers. MM/HFT is just really saying "all high volume firms" while acknowledging that what most

All market makers are HFTs. Most (but not all) HFTs are market makers. MM/HFT is just really saying "all high volume firms" while acknowledging that what most people are talking about when they say "high volume" is market making. Market making software is mostly independent on the market it trades, so its trivial for a market-maker in one product to switch to a similar product.