A codicil is a formal amendment to an existing will - a short document that changes a provision while leaving the rest intact. In Arizona it must be signed with the same formalities as the will itself: writing, signature, and witnesses. A note in the margin or a crossed-out name changes nothing and invites a contest.
Codicils made sense when documents were retyped by hand. Today, most professionals simply restate the whole will, because a will plus a stack of codicils is how conflicting instructions and probate disputes happen.
The better habit: review the will after every major life event - marriage, divorce, births, deaths, a move - and replace it cleanly when it no longer says what you mean.
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Educational information only, not legal advice. Terms have statutory details beyond these summaries - confirm your situation with a licensed professional.