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The suffix is composed of two parts: the series and the vehicle number.
The vehicle number is sequentially assigned within the series, beginning with 1. Therefore, there can never be an OV-100 as it would read "Orbiter Vehicle Series 1 Vehicle 0".
Many proposals to build a second generation of orbiters, externally compatible with the current system but internally new, refer to them as "OV-200" or "OV-2xx" in order to differentiate them from the "current generation", the OV-100s. This terminology is informal, and it is unlikely that any Shuttle-derived vehicle built will be given such designation.
Challenger was originally intended to be used as a Structural Test Article, rather than a flight-capable orbiter; as such, the numbering was changed when she was rebuilt. Enterprise, on the other hand, was intended to be rebuilt into a flight-capable orbiter; it was found to be cheaper to rebuild STA-099 than OV-101, so she remained unflown. The designations were not altered, despite these changes in plans.
In keeping with this theme, other pieces of ground hardware used by the Space Shuttle program have been given official designations; these include:
The 096 and 097 designators were given to structural test articles that were cancelled, but while they exist in some NASA records, the NASA History Office has no official record of STA-096 and STA-097.
One structural mockup has retroactively been given an unofficial and honorary OV-xxx designation:
Note that while Pathfinder is referred to as OV-098, she should not be confused with MPTA-098 - the Main Propulsion Test Article, a steel frame (looking nothing like an Orbiter) used to test the Space Shuttle Main Engines. Pathfinder was originally a "fit check" unit, a simple mockup resembling the production Orbiters in size, shape and weight, intended to be used to ensure that ground handling facilities would be sized to handle Orbiters. She was later rebuilt to resemble them cosmetically, and is currently on display with the external tank used during the propulsion tests (designated MPTA-ET). To further add to the confusion, it appears that Pathfinder was never formally numbered, and the OV-098 designation is both unofficial and retroactive. MPTA-098 was later rebuilt into the Shuttle-C mockup during the 1990s; it is currently in storage at the Stennis Space Center.