Reporting Support for 32-bit Indices
Before DirectX 8.0, vertex indices were restricted to 16-bit quantities. DirectX 8.0 adds support for 32-bit indices. A driver reports support for 32-bit indices by setting the value of the MaxVertexIndex field of D3DCAPS8 (currently also in D3DHAL_D3DEXTENDEDCAPS) to a value greater than 0xFFFF. This field also allows the driver to report that although it supports indices requiring 32-bits of storage it does not support the full range of 32-bit values.
DirectX 9.0 and later versions only.
In order for a driver to expose its Direct3D hardware abstraction layer (HAL) device to applications through DirectX 9.0 interfaces, the driver must set the value of MaxVertexIndex to a value greater than or equal to 0xFFFF.