People (and in some cases, robotic machines) will handle a product many times during the various stages of production, testing, storage, packaging, shipping, selling, unpacking, installation and repair. Because of this, throughout a product's lifecycle there most likely will be unanticipated handling, abnormal use and environmental situations that the product may experience, which may not be customarily foreseen by the product developer. This type of activity can be called unspecified wear and tear and must be a serious consideration within any of the product development methodologies.

This is quite distinct from expected product usage and handling, which is (usually) anticipated to occur within expected environments and which product developers customarily specify during product definition; this can be called specified wear and tear.

Note that both specified and unspecified wear and tear are actually the same phenomenon, but are distinguished by what developers expect and what they don't anticipate, for whatever reasons. In other words, unspecified wear and tear is just specified wear and tear that the product developer didn't think of.

Examples of specified wear and tear:

• Stability of drugs against light and stability of potency while in shipment or storage.
• Capability of lead-acid batteries to provide a useful charge at temperatures below freezing.
• Ability of disk drives to operate within a specified error rate at altitudes above 5,000 meters under extreme but specified vibration.
• Minimization of cell phone radiation into adjacent tissue - e.g., Specific Absorption Ratio (SAR) measured in watts per kilogram.
• Ability of a handheld PDA to 'withstand' a (rigidly defined and validated) drop onto a surface of certain hardness from X meters.
• Testing a portable powered product containing hazardous components (e.g. lithium batteries) to the United Nations Recommendation on the Transport of Dangerous Goods: see http://hazmat.dot.gov/regs/intl/untdg.htm