The comment from EngineType.h is helpful:
| /** Required to optimize mesh in mirrored transform. Double index buffer size. */ | |
|---|---|
| UPROPERTY(EditAnywhere, BlueprintReadWrite, Category=BuildSettings) | |
| uint8 bBuildReversedIndexBuffer:1; |
Looks to be accounting for negatively-scaled meshes and auto-flipping normals on them so they don’t display inside-out, and also for two-sided materials to render without backface culling. It is best to leave disabled whenever possible, as it removes the StaticMesh reversed index memory cost, which is an additional buffer which subsequently lowers the StaticMesh index memory cost. This, in turn, affects the resource size, which includes the sum of those index buffers.