The 30 means 3.0 litres- the combined swept volume/capacity of all six cylinders.

*edit* Google's Gemini states: -

The main difference between the M54B25 and M54B30 engines is the stroke, with the M54B30 having a longer stroke. Both engines share the same 84mm bore, but the M54B30 has a stroke of 89.6mm compared to the M54B25's 75.0mm. This difference in stroke results in the M54B30 having a larger displacement (3.0L vs. 2.5L) and, consequently, more power and torque.

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Wayne
2005 BMW X5 3.0d (b 02/05)
2001 BMW F650GS Dakar (b 06/01)