That is indirect addressing. If the Integer word N10:16 has a value of five, then the first operand in the divide function will be F8:5, and if N10:17 has a value of 10 then the result of the divide function will be placed into N9:10.
You may dynamically change the values in the N10 registers to have the same divide function use different operands on different scans of your program logic.
No. Merely random examples. As I said, you may dynamically change these numbers within your program logic (or from an HMI) to change the operands for the divide function.