Which you click the "Create a Space" button in the sidebar, you get a clear prompt to choose between "Public" and "Private" (and a note that you can change this

Which you click the "Create a Space" button in the sidebar, you get a clear prompt to choose between "Public" and "Private" (and a note that you can change this later in the Space's settings). Personal Spaces are just a special case of a Private Space where you haven't invited anyone else. It's worth calling out this use case separately, as we've otherwise seen users assume that Spaces were only for shared hierarchies, when in fact anyone can create a Space and put whatever rooms they want in it to create a personal system of organization, even if those rooms are already in other Spaces. If you want to turn your Personal Space into a shared Private space, you can invite people via the button on the Space's summary page. If you want to open it up to the Public, that's in the settings menu accessed from the same page.