If you get the Centris 650 with CD configuration, you are getting a Mac with
a 68RC040 processor that has built-in math coprocessor support.  My 
understanding is that the "optional fpu" refers to your option of purchasing
the Centris 650 4/80 without FPU OR one of the other configurations WITH FPU.

Apple does not offer an upgrade from the non-FPU system to become an FPU
system.  And, it is unclear whether the '040 processor on the non-FPU system
(a 68LC040) can be replaced with a 68RC040 supplied by another vendor.
Apple did send a memo out at one point sating that the Centris 610, which ONLY
comes with a non-FPU 68LC040 processor CANNOT be upgraded to support an FPU -
the pin configurations of the two chips apparently do not match so you cannot
swap one for another (again, according to Apple's memo).

Hope that helps.