Retention means that a specific volume of water is retained on the roof for an indefinite period of time.  Detention means to temporarily detain stormwater before discharging it as runoff.

RoofBlue is intended for use as part of a water detention system, whose main function is to delay and control the flow of stormwater runoff.  This delay allows more time for municipalities to treat runoff, which helps to prevent stormwater and combined sewage systems from overflowing (untreated) into local waterways.