Every formula, note, photo and visit stays on the client's record, so the next appointment starts where the last one ended.
Write the things future-you will not remember: the exact formula and developer, timing, what you changed from last time, and anything they said about how it wore. Ten seconds at the end of an appointment saves a re-consultation at the start of the next one.
These records are yours, kept under your account. What a client carries between professionals is their own Beauty Passport, which they control and share deliberately.