I've fleshed out the help page with some explanations of how trip privacy works, as well as some information about exactly where you can expect your data to show up — will it appear in search engines, for example?
I realise that confidentiality of information is a huge issue in the caving community, and it has always been my ambition to ensure that all data submitted to caves.app is totally private until the user specifies otherwise. It goes without saying that it is unlikely that anyone would want to use a website where they have concerns about the privacy of their data, or are confused about who their data is visible to.
Everything on caves.app is totally private, by default. It will always remain that way, even when new features are added. All profiles are, by default, visible only to the user they belong to. Trips, by default, inherit the privacy settings of the user profile — so they are private too. If a user does choose to make their profile visible, either to their friends or the general public, they will have to opt-in (via their account settings) to have their trip notes displayed to other users. I don't want anyone to be displaying any data to the world by accident.
For further information about exactly how the privacy system works, have a look at the new items on the help page.