Factory Mutual is a large insurance organization. Their specifications have been widely accepted as fundamentally sound and protective of buildings. Other insurors require compliance with FM, mostly for commercial/industrial buildings. FM is usually at least as stringent as the building codes.
Ron has correctly stated that Factory Mutual (FM) is a large insurance organization that drafts standards that try to ensure that the property owners building will survive most hazards. The class the OP is refering to "I" or "1" - 90 and refers to an acceptable wind pressure of about 45 lbs per sq. Ft. and then doubled and applied to the field of the roor surface with higher numbers at the edges and ridge and even higher numbers(pressures) applied to the corner zones. Recently (last ten yeats or so)the system has been based on the IBC Code with Importance factors of 1.15 (essential facility) applied.
They also have snow loads with a coef. of 0.9 * the Ground Snow Load instead of 0.7 * the GSL to get the Flat Roof Snow Load.
Their rules apply to fire rules, sprinkler systems, Hail, seismic, etc., etc.
You can download there design rules from their website which you can get by searching for Factory Mutual Insurance.
Their rules only apply if your owner wants to be insured by their company or one that uses their standards.
Many Architects call out their specifications rightly and wrongly.
