Mercury Outboard series

[Mini GT]

Folks, I need an education on the 2.5l Mercury outboard models. What are the subtle technical differences between the following Mercury models.
XR, XS, EFI, Pro Max, OPTIMax and anything other models I misses out in the line up.

Thanks

[rock]

Way too many to list. The xr2, xr4, and xr6 were fishin motors 2.0, 2.4, and 2.5 respectively. The xs motors are factory built by the racing division. EFI is simply electronic fuel injection. Promax are "high performance" and the Optis are direct injection. Hundreds of other differences.

Rock