The RPF stands for Rehabilition Project Force.
Supporters see it as a program designed for Sea Org members who need specific areas of their lives handled so they can work and live to their fullest. It is a strenuous program and people often speak down upon it when they are unable to finish it or have family members who are in it and complain. However supporters claim it gets fantastic results and the people who complete the program (which consists mainly of a lot of hard work, running, lots of studying, and scientology auditing) allegedly continue on with life feeling very good about themselves and are able to live life to it's fullest.
Opponents see the RPF conditions as similar to prison conditions and slave labour. It appears that DM (David Miscavige) runs at least one other unofficial prison called "the Hole", see Debbie Cook - 45 Days in the Hole.
Some, perhaps many Freezone Scientologists are critical of the Rehabilitation Project Force as used by the Church of Scientology as is the International Freezone Association. The Rehabilitation Project Force is seen as one of many abusive systems within the COS that David Miscavige permits.
The International Freezone Association put up a informational website called scientologistsfreezone.com in which detailed information demonstrating the differences between the �official� church and the Freezone were outlined. Also detailed are some activities the church engage in called the Rehabilitation Project Force (RPF), a way of utilizing labor at a minimal cost, and the cost of human suffering this has had on the members in the RPF. 
In the quote below Marty Rathbun compares an American prison with what he calls the “DM Hole at Int” which appears to mean the RPF or something similar. Ratbun may be refering to a time when many high ranking Scientologists were confined to conference rooms and required to sleep under desks but those confined feared the RPF and felt the RPF would be worse. 
And this lead to the lessons. First, the OPP Hole was a far more humane environment than the DM Hole at Int. In OPP, the First Amendment right to freedom of expression was not suppressed, not by the authorities nor by the prisoners. Even when my speech offended some no one resorted to nor even threatened violence.
When I was quietly lamenting about having no means to find Mosey, a particularly scary looking twenty-something kid with dreds approached me. He was up on his third drug charge (guaranteeing hard time in Angola and guaranteeing disqualification for any meaningful employment for the rest of his life). He gently took me aside and two-way commed me into some rationality. I then took interest in his story and those of others similarly situated. I returned the favor that had been done for me to several others. There wasn’t a single criminal in that cell whom I did not get along with and find something to like about. Every conversation I had was deeper and more meaningful and more caring than any conversation that was ever permitted in DM’s Hole.
Other Hole comparisons:
There were no attempted, let alone coerced, confessions in the OPP.
There was air conditioning in the OPP, something sometimes cut off as punishment in DM’s Hole.
There were some tough guards in the OPP, but never were they cruel or abusive as is required in DM’s Hole. And even one of them took interest in my personal dilemna.
There was no requirement to salute anyone, much less dogs.
There were no humiliating games like “musical chairs.”
There was no talk about people’s mothers – in fact, I got the distinct impression that was the one thing that would permit unbridled violence in OPP. I compared that to DM’s obsession to slander my mother who passed away nearly fifty years ago.