An oft quoted phrase in regard to suitability of alternative devices, but exactly what does it mean?

In the most extreme definition, it means alternative devices are exact equivalents, impossible to distinguish from the original devices. Regarding electrical performance, this is harsher than is ever needed as a practical working definition, and being able to pass the original manufacturer’s spec is all that is required. Indeed, it may well be that replacement devices out perform the originals in terms of tighter tolerances and reduced manufacturing spread. Replacement devices also need to physically match the PCB footprint of the original devices, but once again it may be acceptable to slightly exceed original dimensions if the device will fit in the space available in a particular installation

There are circumstances where a more relaxed definition will give an acceptable solution. For example, where an obsolete device is used in more than one applications, there may be some instances of its use that require only a true form, fit, and, function match for a subset of its features. If we can create a new device matching that subset, then working old devices can be swapped out for use in more critical areas.