I think it is a left over from when BMW only had one version of each motor and they stated the engine capacity (in litres) to allow people to show whether they had the larger (and more powerful and more expensive) engine. Now they also seem use it to distinguish between less and more powerful engines of the same capacity.
I think the 3 litre in-line petrol six currently comes in 3 different versions, each with their own "displacement" number. IIRC it goes something like this (but I might be wrong on the details, I've never really taken much notice as I'm unlikely to ever buy a new BMW):
Std (non-turbo) "x30" (230 hp, also badged as "328" in current 3 series ?)
Normal Turbo "x35" (300hp)
Hi-po turbo "x40" (just to be confused with their NA 4.0 litre V8 ?)
2017 MY18 Golf R 7.5 Wolfsburg wagon (boring white) delivered 21 Sep 2017, 2008 Octavia vRS wagon 2.0 TFSI 6M (bright yellow), 2006 T5 Transporter van 2.5 TDI 6M (gone but not forgotten).
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