The "Multiprosessing" in this case refers to Symmetric Multiprocessing (SMP).
This is when you have more than one physical processor on the motherboard.
The first number in the second section of the Intel model number tells you the maximum SMP configuration the processor can be part of. So for example, E5-1650, it's a "1" and with E5-2643 it's a "2".
The E5-1650 cannot be used in an SMP configuration, and the E5-2643 can be used in an SMP configuration with (up to) 1 other processor (2-way SMP), and something like the E5-4620 can be used with up to 3 other processors for 4-way SMP.
Here's an link to a comparison of these three CPU's at Intel's ARK (see the line titled "Max CPU Configuration"):