Since I am not a public-safety radio industry professional, I won't speculate about which official terminology the public-safety radio industry would use to describe System 9. But I can comment on programming a scanner to track System 9, which I assume is the purpose of your question.
System 9 has channel 20 as 851.50000 MHz, a splinter frequency. If you do not program the system as splinter in your scanner, the scanner will interpret channel 20 as 851.51250 MHz, it will tune to 851.51250 when you want it to tune to 851.50000, and you will miss any traffic that gets assigned to channel 20.
In short, program System 9 as "splinter" in your scanner to ensure you don't miss transmissions on 851.50000.