What does "part of a space" mean in terms of the space hierarchy?

[MadLittleMods]

Link to problem area:

Rooms can additionally claim to be part of a space by populating their own state with a parent event.

-- https://spec.matrix.org/v1.4/client-server-api/#mspaceparent-relationships

Relevant MSC1772

Issue: What is wrong? How can we improve?

Confusion as experienced #synapse-dev (most of this is cribbed from @reivilibre's comments),

It's not clear what "part of a space" means here.

• Are rooms, with m.space.parent → some space, meant to show up in the space hierarchy for that space?
• Or is that only for rooms which are a m.space.child of the space?

Element (the client) will display the room in the active space if it has a parent of that active space, but not in the hierarchy.

rstrst room with m.space.parent shows up when viewing the space	rstrst room with m.space.parent does not show when viewing it in the space hierarchy

[deepbluev7]

In my opinion it should show up in the hierarchy, but only if the user is a member of that room already. Otherwise you leak hidden rooms. (Also of course the powerlevel check has to pass.)

[clokep]

Are rooms, with m.space.parent → some space, meant to show up in the space hierarchy for that space?

Do you mean "in the response to the /hierarchy request"? Then the answer is no, they are not.