production support and maintenance is the stuff you do to manage software defects so that operations continue per the specifications. sounds good, but you would think that, with proper funding and assuming that software was functional in the first place, that the majority of defects would be resolved in short order and those charged with “production support” would soon have nothing to do.
User behavior is the main cause of and overloaded production support crew. Applications are never stable, new user requirements, misset expectations, and poor change resourcing keeps production support on their toes. A shifting architectural foundation of patches, upgrades, license constraints, and network reconfiguration keeps tickets flowing in to the service desk.
And don’t forget about the ever threatened maintenance windows and do-not-touch timeframes. These are constantly crimped by changes in use geography and business calendars. There’s little hope of profiling these effects in the original application requirements.