PFL - Procedural Fairness Letter is what you get before they reject you... It works like this, they'll analyse your file and make a decision, if the decision is not to give you a NoM, then you get a PFL.
Now, you may be asking, why would you not get a NoM... The two main reasons I've seen here are:
- Candidate applied as a CEC, but OINP's calculation of his Canadian work experience is less than a year, therefore he hasn't met the basic criteria for CEC
- No intent to reside on Ontario, this is when you've failed to convince the officer in charge of your application that you'll move to Ontario if they nominate you, usually you'll get an additional docs request asking you to submit your ties to Ontario before this happens.
Once you get a PFL you get given some time (I think it's 30 days) to reply addressing the issues they've noted, so for example, suppose you didn't convince the officer of your intent to live in Ontario, then you get given ONE more chance to convince them. Your response will be looked at by another senior office with fresh set of eyes, if he's convinced then you get the nomination, if not then you don't.
PFL is seen as a fair rejection mechanism as it gives the candidate a chance to defend himself before the final decision is made.
Now, as for when you can get a PFL I'm not sure... For me it would make sense to be on the decision phase, but perhaps some other members with more knowledge or experience specific to PFL can help you.