By channel space do you mean in a radio receiver where this PLL is providing the local oscillator frequency?
Many PLL chips can only divide by integer numbers. So lets say you need a 50 KHz channel spacing in the PLL microwave output frequency, such as 1000 MHz, 1000.05 MHz, 1000.1 MHz..., one way to do this is to have a reference frequency of 50 KHz, and have a divisor N operating on the microwave output, where N is 20000, 20001, 20002.
You could just as easily have a reference of 25 KHz, and use an N divisor of 40000, 40002, 40004. If you did this, the microwave phase noise would be 6 dB worse over the original example above for frequency offsets inside of the control loop bandwidth. Sometimes that additional phase noise is a problem.
Some of the newer PLL chips use fractional N dividers, so you can use a higher reference frequency and still get the channel spacing that you need. Lets say you once again wanted 1000, 1000.05, and 1000.1 MHz channels. You could choses a reference frequency of 800 KHz, and use the following divisor N= 1250, 1250 1/16, 1250 2/16.
Does that help?