Simple question about using flat planes as geometry

[Alexx31]

Hello,
I remember I learned once, that I have to use only objects with Tyflow, which are thick meshes and also "watertight" closed.
But actually, Tyflow works also with flat planes.

Which rules are true now?

Thanks

Edit: -> Actually I know from working with cloth planes, that this isn't true.
But there have been some special rules for fracturing, at least a year ago.