I’d say the company marketing the blank makes designation decisions based on what makes sense to them. In Batson’s case (per their product descriptions, not my words) they designate as follows for a 9’ 5 wt.
RX-6, 33 mil modulus (standard) is F905
RX-7, 43 mil modulus (intermediate) is IF905
RX-8, 54 mil modulus (high) is XF905
They could have designated them anything they wanted to though. Other manufactures may use similar designations while others something quite different. Trying to avoid the modulus designation discussion, I’d guess that in Batson’s case “I” is for intermediate, “F” is for fly.