All fiction requires some suspension of disbelief, but speculative fiction is distinguished by a greater cognitive leap required by the incorporation of story elements that significantly diverge from our understanding of the known world. Moreover, the type of the break from reality defines the sub-genres: fantasy stories violate the laws of nature, alternative histories contradict the historical record, and science fiction challenges our technological expectations. In so doing, speculative fiction both enables readers to escape farther from reality, and equips writers with unique settings in which they may convey original stories.

You used “concept,” but I prefer “setting” to capture this distinction. Speculative fiction differs functionally in that the setting diverges significantly from our understanding of reality. Realist fiction, however, showcases that which is possible in our known world, and does not require the reader to set aside some boundaries of reality to explore the unknown.