There are a couple of things that you should know about sheeting when selecting an outdoor sign.  All sheeting is not created equal.  Sheeting is the material that covers the substrate of the sign.  Essentially it is the background the the letters or graphics will be laid onto.  On a "speed limit" sign for example it is the reflective white background on which the black letters/numbers are placed. 

There are four common types of reflective sheeting.  In order of reflectivity from least to most reflective they are: Engineer grade, Super Engineer grade, Hi Intensity, and Diamond grade.  Depending on the nighttime visibility requirements of your sign you'll want to choose the appropriate reflective sheeting as this will effect the end price. 

Engineer Grade sheeting has reflectivity visible from about 500ft and is the least expensive and is commonly used in street signs and municipal signs. The step up is Super Engineer which is visible from about 750ft. Super Engineer will be the minimum required sheeting for cities and municipalities nationwide starting in 2012.  Hi Intensity sheeting will be visible from at least 1000ft.  Diamond grade sheeting, the most expensive, is visible from more than 1200ft and is commonly used on road construction signs and interstate highway signs where the higher speeds require legibility from longer distances. These reflective values (distances) are based on an average vehicle's low beam headlights making the sign visible from said distance.

Take this into consideration next time you need a reflective back sign. It can save you money by not buying an excessive sheeting or having to rebuy a sign because it doesn't meet federal standards.