The newest catalog the controller has seen: its own copy, to read and not change. Null until the first load.
The held layout was changed or deleted in another session, or by another
control on the page, since this controller last opened or saved it, and
a write into it was refused or a reload found it so. save() and
autosave are refused until overwrite(), saveAs() or open() settles
it; a reload clears it only when it finds the layout as this controller
left it.
The target differs from its layout. Checked when changes settle and by flush(); false with no layout.
Why the last operation or autosave failed; null after a success.
The saved layout the target was opened from or last saved to; null for none.
The revision the next write into the held layout is checked against;
null until the first load. It trails catalog.revision while the held
layout conflicts, so that write is refused rather than laid over a
version nobody here has seen.
The target is suspended (LayoutTarget.suspended): autosave waits and open is refused. Since 2.5.10.
A layout operation (anything but an autosave) is queued or running.