Rotation is one of those instructions that sounds trivial and then quietly stops happening. You are told to vary the site, you intend to, and three months later you notice you have been using roughly the same patch of the same side because it is the easy one to reach.
The reason to rotate is that repeatedly injecting the same small area can change the tissue there over time, and tissue that has changed does not absorb the same way. That is not a cosmetic concern. It is a consistency concern, and inconsistent absorption looks exactly like a dose that has stopped working.
Rotation only works if it is recorded. Human memory for this is genuinely poor, because each injection is unremarkable and there are only a few candidate sites. If you ask yourself on a Monday which side you used last Monday, you will guess, and you will guess the same wrong answer repeatedly.
The practical fix is a single extra column. Site, written the same way every time. Left abdomen upper, right thigh outer, whatever vocabulary you like, as long as it is consistent enough that you can scan a column of twenty entries and see a pattern. Free text is fine. What kills it is describing the same place three different ways.
A fixed order helps more than a random one. Cycling through your available sites in a set sequence means the log tells you where you are without you having to think, and any deviation is visible at a glance.
Note anything unusual next to the site entry. Lasting soreness, a lump, unusual redness. On its own a single note means nothing. Three notes on the same site across two months is the kind of pattern worth mentioning to a prescriber, and it only exists if you wrote all three down.
Site sits next to date and dose in my log in Peptide Tracker & Calculator, and the glossary covers the terminology if the paperwork you were given assumes more than it explains.