Possibly a Department of Defence Designation allocated to but not used for the Sprint anti-ballistic missile at the time the Sprint II was considered. Another version is the designation was not used to avoid confusion with the old-system IM-99 designation for the Bomarc missile.
Historical Essay © Andreas Parsch
On 27 October 1972, the designations XLIM-99A and XLIM-100A were reserved by the U.S. Army for unidentified missiles. Unfortunately no further details are available, but the reservation was apparently not followed-up by a formal request for nomenclature allocation.
The "XLIM"-prefix (Expermental Silo-Launched Interceptor Missile) of course points toward the U.S. Army's anti-ballistic missile (ABM) program of the late 1960s and early 1970s. Apart from the LIM-49 Spartan and Sprint interceptors, which became briefly operational in 1975, there were also a number of experimental and projected missiles, including HIBEX, Squirt and Sprint II. However, the HIBEX and Squirt are no candidates for the LIM-99-100, because these programs were completed by 1972.
While it is of course possible that one of the numbers 99-100 was reserved for Sprint, there are some reasons against this assumption:
There is, however, at least one possibility. In May 1972, Martin Marietta was awarded a contract for an improved Spint development called Sprint II. There were now two missiles named Sprint, and someone in the Army's ABM office might have deemed it necessary to allocate different designations to avoid any confusion in the future. If higher authorities didn't agree with this necessity, this would explain the lack of any written request.
Note: I've been told by one individual, that the reserved XLIM-99A designation was not used to avoid confusion with the pre-1963 IM-99A designation of the CIM-10 Bomarc missile, and that XLIM-100A was reserved for the Sprint. However, this does not explain why two different numbers were reserved when only one was needed for Sprint, and why the LIM-100 nomenclature for the latter was never formally allocated and-or publicly announced.Specifications
The missile project, for which the LIM-99 designation was reserved, has not been identified so far.Main Sources
 Department of Defense Missile Nomenclature Records
Status: Cancelled 1975.