glActiveTextureARB.3G man page
glActiveTextureARB — select active texture unit
void glActiveTextureARB( GLenum texture )
Specifies which texture unit to make active. The number of texture units is implementation dependent, but must be at least two. texture must be one of GL_TEXTURE_ARB, where 0 ≤ < GL_MAX_TEXTURE_UNITS_ARB, which is an implementation-dependent value. The initial value is GL_TEXTURE0_ARB.
glActiveTextureARB selects which texture unit subsequent texture state calls will affect. The number of texture units an implementation supports is implementation dependent, but must be at least 2.
Vertex arrays are client-side GL resources, which are selected by the glClientActiveTextureARB routine.
glActiveTextureARB is only supported if GL_ARB_multitexture is included in the string returned by glGetString when called with the argument GL_EXTENSIONS.
GL_INVALID_ENUM is generated if texture is not one of GL_TEXTURE_ARB,
where 0 ≤ < GL_MAX_TEXTURE_UNITS_ARB.
glClientActiveTextureARB(3G), glGetIntegerv(3G), glMultiTexCoordARB(3G), glTexParameter(3G)