On that page it doesn't seem like Level B has a specific back flight acro requirement, but Level C has RO BHS to salto, so I wonder. Of course, individual gyms/coaches will probably have their own requirements for the elements they want to see as well. My guess is that the intention of level B, based on the requirements for all events, is to provide a level "between" level and level 6, while level A is "between" 4 and 5. That way girls who aren't quite progressing in the USAG system due to certain problems (for example, level A is "mostly harder" than USAG level 4 by requirements, but does not require a kip on bars - though does lower SV for pullover - much less two). So a girl missing the kip out of 4 could compete this level before doing USAG 5, say in the spring with optional routines. That's not the only way MDL can be used of course, but just one way based on the way the requirements have been set. I'm not sure how mobility through the levels goes, I'm guessing there is some mobility.
Also since these are listed as skill requirements I doubt substitutions are allowed. You can see where some substitutions are noted, i.e. kip or pullover with lower SV, cartwheel or BWO in level A beam. If not noted, any substitutions won't fulfill requirements. Also, because level A spells out the acro skills (front handspring and RO BHS), my initial assumption would be those are intended both as requirements and acro difficulty restrictions, so a front tuck would not be allowed (would void the routine or receive a significant deduction). Otherwise I would guess they would leave open "acro skill with flight" but that doesn't seem to be their intention at that level.
