For US-issued HSBC cards, it's normally $1-1point-1Avios. I think mileage transfer is only possible for Premier accounts.

Since someone mentioned HSBC in HK, I'd point out that Avios redemption is also available for RewardCash, but only for Premier accounts. (Not sure how long this has been available.) This is normally not as good as the rate offered by the BA-branded card in town because it's only 1 Avios per HK$15 (technically it's HK$250=1 RewardCash=15 Avios). But their ongoing 6x promo means that for your selected spending category it becomes HK$250=6 RewardCash = 90 Avios, or HK$2.77 per Avios, which I think is more generous than just about any other earn opportunity anywhere.