SSO = simultaneous switching output
SSON = simultaneous switching output noise
There are two cases. Take an example of 8bit output
(1) All 8-bit switching with the same frequency and toggle 1 and 0. You need to measure the overshoot/undershoot/+-ve ringback/-ve ringback
(2) 7-bit switching with 1-bit quiet at 1 and 0 for two cases. Measure the quiet bit for +ve bounce and -ve bounce.
Why we do that? Because we need to understand if the bouncing level will falsely trigger the logic input. Say if you have TTL L input is 0.8V. If your bouncing is higher than 0.8V, you will get wrong logic trigger.
When u do simulation, remember to include RLC package model for power/ground and the IO pin, otherwise, it's not realistic. Also better add transmission line model when doing simulation. All need to add realistic output loading