If it helps, SPD usually means Supplementary Performance Data. It's similar to saying 'typical' or 'the most likely result' when an attempt is made to verify the performance.
When you have to use words like Usually and similar to describe it, seems it's not common. From the context, I would have assumed typical but then why use an acronym? Searching your exact phrase on Google, the closest match was a site talking about laws in Europe.
Proper documentation will define acronyms before using them. Now how fast the 2-port system will scan 1601 pts with a 10Hz IFBW or if it is a 4-receiver system, still up for grabs. Pretty sure I can count on the encoder skipping and randomly incrementing/decrementing when turning the knob in one direction... Oh right, turn it slower or use the keypad.