An email to buyers of the Bloks said of Reap-X
"The Office of Naval Intelligence, commonly referred to as ONI, is the intelligence service branch of the UNSC. While ONI serves various functions and operates under a multitude of subdivisions, the existence of the Reverse Engineering and Prototyping–Xenotechnology department, acronym REAP-X, is unknown outside the ONI division. Working under the ONI umbrella, REAP-X develops concepts, technologies and applications that are often field tested by ONI via other military branch deployments.
The REAP-X department evolved from smaller splinter groups created during the Human-Covenant War and helped funnel weapons and shield technology to human forces via Misriah and other firms as the war came to a close. Its big breakthrough however, was a Forerunner slipspace engine discovery that created the basis for the engineering concepts that helped complete the Infinity project. With unlimited resources, funding and attention from top brass the department houses many variants of never before seen military vehicles developed for special missions."
We're light on real details but I'll speculate maybe the VTOL was cut from the campaign of Halo 4?