Not sure. I had one retired AA CA write me a letter. I uploaded it and then he sent it in to the Pilot Recruitment folks. It appeared on the bottom of my profile a few days later. I would say that having that internal go through AA into PilotCredentials would be better than having a letter that you upload yourself that nobody is going to read. Bottom line is that nobody really knows except for HR and they aren't going to tell anyone.
The way that I have heard it explained at another airline that uses PC is that they state the requirements they want, PC spits them out, and only then they are reviewed by a human prior to being invited for an interview. If there are too many, they go back and make the requirements more stringent and crunch them again. If there are not enough, they lower the requirements and crunch again.
Of course, networking is king...military guys I know that have been hired have had multiple internal recs and many have gone to job fairs or met chief pilots. I would assume the same applies to 121 only guys.