Pressure rated vs non-pressure rated is really a question of how a pump of a given power rating is optimized (I use the term power rate very lossely here knowing that some pumps are designed more energy efficent than others).
Pressure rated (and again I use the term loosely here) pumps are normally optimized to be able to operate into larger heads or pressures and in turn have lower maximum flow rates (the old adage you don't get something for nothing applies in a surprising number of situations). Example uses - pushing water through fine filters, operating into long pumbing runs, push water up higher elevations, etc.
Non-pressure rate pumps are typically the reverse, they are designed to give larger flow rates but limited in terms of the heads or pressure they can operate into and still achieve these flow rates. Example uses - internal/external circulation pumps that move water from one region of the tank to another.
Depending on your requirements you may need both high flow rate and good pressure capability in a pump and while you can get this you will tyipically have to go to a larger pump (i.e., higher wattage and/or horsepower rating) to do so with all the down sides associated with this (e.g., higher costs, noise, heat, etc.).
While there is no set rule, I tend to feel that a pump with a max head of 15' or less is best suited for circulation type duties while for higher pressure requirements you should look for max heads of 20' or more in a pump. Milage may vary as they say and depending on your specific requirements may find that you may need more or less than indicated. One of the reasons I wrote these articles on pumps was so folks would be able to use some relative simple techniques to help determine if a particular pump is appropriate for their needs or not.
Hope this helps, RN
