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Indeed, fixrouter4400 is right. In Virtuoso Layout you do the layout just looking at the schematic, but it (the schematic) does not influence the layout tools itself.
In layout XL, you can import the netlist from the schematic into the layout (generate from source). That is the first difference. But there are more. Virtuoso XL offers many automation features, like: custom placer, custom router, online DRC and many others features.
So why don't we all, layouters, use the XL? Because it is not (yet) aware of mathing techniques, power rails and others, but only matching is enough as a proof to NOT use it in general, because how can you do a layout without mathing?
Layout designs are defined: what percent can you alter the ideal schematic by bringing it into the real world. With XL, in analog design, you destroy it completely!
In extremely large designs maybe it is usefull.
Anyway, that's only my opinion.
IF anyone has better arguments for XL, please write them down.