H.D.
The IBC always had the limit on fixed seating, a modification trend has been happening in enough where by the adopting agencies have been modifying the IRC to include fixed seating that during the CTC's review of climbable guards reviewed documentation showing seating surfaces are a point were injuries were related and after lengthy debate added the same requirements to the IRC code, that were in the IBC.
Seeing a possible conflict with the wording both the IBC-MOE & the IRC committees (passed the proposals in 2009) and requested during the committee hearings that the CTC define a distance and better measurement placements and submit future proposals to clean it up.
That wording was added in to the code in the 2012 IBC & IRC during the committee hearings, later on at the Dallas finally action hearings, the IBC portion was passed and the IRC section was modified by another public proposal that failed committee, but passed final action, in other words changing the wording again. But that does not come about in till 2012 is adopted.
As for a measuring point, though the 2009 code(s) don't say fixed seating within 36" of the guard, it does say within 22 inches in the 2012 code, therefore a code official IMO can reasonable apply the requirement when the fixed seating is within 36" of the guard in the IRC & within 42" in the IBC and not require the restriction past that point, pointing to a published model code not yet adopted, that specifically was written to clarify the question in concern and uses a reduced trigger point.
The reason I suggest the distances I do is the arc length matches the required guard height and weather it is on top of the fixed seating or away from the fixed seating the distance is still in place.
I will post the 2012 IBC measuring diagrams later on.
Tom