[5] Memorex in 1968 was first to enter the IBM plug-compatible disk followed shortly thereafter by a number of suppliers such as CDC,[6] Itel, and Storage Technology Corporation.
Thus, it is important for customers to understand the difference between a "bug" and a "feature", where the latter is defined as an intentional modification to the previous system (e.g. higher speed, lighter weight, smaller package, better operator controls, etc.).
Some non-conforming units may be re-packaged or modified to meet plug-compatible requirements, as where an adapter plate is provided for mounting, or a different tool and instructions are supplied for installation, and these modifications would be reflected in the bill of materials for such components.
Similar issues arise for computer system interfaces when competitors wish to offer an easy upgrade path.
In general, plug-compatible systems are designed where industry or de facto standards have rigorously defined the environment, and there is a large installed population of machines that can benefit from third-party enhancements.