The main difference between resident set management and page replacement policy:
In resident set management the operating system must decide how much main memory to allocate to a particular process when that process is swapped in. This can be a static allocation made at process creation time, or it can change dynamically.
In page replacement policy when the memory is full, a decision must be made as to which page or pages are to be replaced. The page replacement policy determining where the page should be load, and how the page load, and what type of algorithm should be used to load a page.