Really basic question here: can someone please explain what "trim" refers to with regards to regulators? And, is it the same as the percent capacity?  

Its the special little goodies inside that change or somehow modify the pattern of flow through the valve to get a particular desired characteristic, such as low noise, reduced cavitation possibilities, etc.

The trim pertains to components such as the valve plug, seat, and associated components.  In the case of some "self-contained pressure-regulators, few options exist besides the actual regulator model selection.  For example, perhaps the valve plug in a Fisher 620 or 630 "Big Joe" has a Teflon disc that seats against a nozzle similar to the seat on a pressure safety relief valve.  However, with control valves there are many selections for metals, soft parts and the method that the pressure drop is made.

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