International code now requires a permanent label showing the duct run's equivalent length. The general idea is to help mitigate fire risk by helping match dryer manufacturer specifications for length of run limits. In some instances dryer manufacturer specifications allow longer runs than is laid out in standard code. Armed with this information, it is easier to use the manufacturer's allowable run length instead of following a more restrictive requirement.
Actual IMC 504.6.5 and IRC 1502.4.5 Code for Labels
Length Identification. Where the exhaust duct is concealed within the building construction, the equivalent length of the exhaust duct shall be identified on a permanent label or tag. The label or tag shall be located within 6 feet (1829mm) of the exhaust duct connection.
Download the code requirements: (PDF)2009 International Mechanical Code
2009 International Residential Code