INFI is Busse's signature steel, only used by the Busse family of companies.
SR101 is a version of 52100 steel, with Busse's exceptional proprietary heat treat applied. It is a cheaper steel, so provides a less expensive version of the legendary Battle Mistress than the previously offered INFI models. But it isn't a worse steel; it is less tough, but still very tough; whereas has better edge holding than INFI under most uses (but INFI still has better edge holding when chopping hard things). The main difference to INFI is that SR101 is prone to rust, though isn't particularly bad in this area.
Busse switches between offering models in these two steels (and sometimes others).