Here's a spreadsheet that shows how the source and RxLO PLLs are programmed to use different harmonics.?? I've also included \the source code from si5351.c that actually implements it.
You can enter a desired frequency in the yellow highlighted cell to see what happens.? That frequency is compared against the threshold (and multiples) to decide which set of harmonics to use (highlighted in green, according to how it works out), and in turn, that determines what the PLLs in the SI5351 are programmed for.
There's a matrix that compares the Source and RxLO frequencies, and all the harmonics, and you can see that only one of them is 5kHz (which is highlighted in red).
You can also change the "threshold" frequency (normally 300 MHz).
