I'm not sure exactly what they call it but consider it a design number. The old style X5, years 2000-2006 was the E53. The new X5 is the E70. As BMW develops new models or complete restyles for current models they give it a new number.

I think someone posted the 2008 brochure yesterday.

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Current:
2016 X6 ///M Black Sapphire Metallic