[detaro]

two parts: purpose (e.g. GDPR excludes household activity, which a private journal of "everyone I talk to" would be if it's only for private use) and structure (with at least for GDPR a neatly kept notebook/ledger possibly already being organized enough to qualify). Although the observations are not "theirs" as in "their property", they merely gain rights against you to obtain information about them and copies, and rights to control your usage.