Storage in general can include aircraft not in active service for reasons such as: after lease return and awaiting new lease term, damaged after an accident, airline decides to remove aircraft from service for some time (capacity cut, restructuring etc.), aircraft are grounded (airline bankrupt or fleet grounding such as 787 safety issues in the past)
Storage does NOT include aircraft under normal regular maintenance or short-term removal from service (say one or two weeks).
When we have an aircraft that has been parked continuously for 5 years (not always in the same location) we will review the aircraft on a case by case basis and if we can find no evidence of it being readied for service or in a continued active storage program then we will deem it as being permanently withdraw from use at the 5 year parked interval. In the few cases where an aircraft then does return to service any time after, then we rescind the permanently withdraw from use and return it to a parked status prior to its return to the air.