Pre-filter and post filter are the two most common strategies for restoring the volume removed by ultrafiltration. Renal replacement fluid - taken literally - is the volume of fluid, similar to extracellular fluid in composition, which is used to replace the volume which is removed by ultrafiltration. All or only some of the volume can be replaced, which is determined by your desired net fluid removal rate.
The physical effect of pre-filter and post-filter replacement fluid has been explored in Question 5.1 from the first paper of 2009. This chapter represents a brief synopsis designed to remind the Fellowship exam candidate about these half-forgotten concepts. For a more detailed review of these principles, an extensive rambling digression is also available ("Pre- and post-filter replacement fluid") - it is aimed at some sort of theoretical midpoint between the Part I and Part II exams.
In summary, post-dilution replacement results in a higher clearance of middle molecules (those dependent on convection); the clearance of small molecules (those dependent on diffusion) is greater because a higher concentration gradient is maintained. Also, less replacement fluid is required, which affects the cost of treatment. However filter lifespan might be shorter because of higher end-filter haematocrit. In pre-dilution the filter lifespan might be longer but the clearance of solutes might be slower, because of lower concentration gradients (even though the theoretical maximum dose of dialysis is much higher).
The strategy of giving the replacement fluid before the countercurrent filter has several positive and negative implications. Ronco, Bellomo and Kellum's Critical Care Nephrology (2009) within its cavernous interior contains a chapter (number 249, on page 1370) by Zhongping Huang et al, dedicated solely to the discussion of pre and post dilution. The time-poor candidate would be well advised to steer clear of this chapter, and of this textbook as a whole. In its effect on the trainee, it functionally resembles the Necronomicon. To prevent an epidemic of madness among the CICM exam candidates, I have scraped together some salient points from the abovementioned chapter into a table of comparison.