Own an aircraft for long enough and you’ll eventually be faced with a modification or repair that’s considered a major alteration. In FAA speak, that’s an official term and a big deal that almost always requires more than a logbook entry for returning the aircraft to service.
In a regulatory climate that’s been somewhat relaxing (especially in the avionics market) one of the most common and difficult questions to answer is whether a given repair or modification needs a special signoff. If you attempt to interpret the regulations, the requirements are more than paperwork. The two pertinent FARs involved are Appendix A to Part 43, which defines major repairs and alterations, and Appendix B to Part 43, which covers the recording of those major repairs and alterations.