Critical Success Factors

If you regard quality as a critical success factor for customer retention then you would set up a maximum failure rate that should be achieved. Then you monitor actual failure against that limit, That is a monitoring CSF as you are measuring current performance to monitor it.

If you regard growth in customer numbers as a CSF then you would set up targets to be achieved in future months/years. That is saying that customer numbers must be build up and those targets would be building CSFs.

Roughly: monitoring = keep doing what you are doing to the right standard. Building = keep doing better so that future performance improves.