Complex sentence: A specification stemmed from an Operational Requirement, abbreviated "OR", describing what the aircraft would be used for-this in turn led to the specification itself, e.g. a two-engined fighter with 4 machine guns.
Split into simpler sentences: A specification stemmed from an Operational Requirement, abbreviated "OR", describing what the aircraft would be used for. This in turn led to the specification itself, e.g. a two-engined fighter with 4 machine guns.