The default value for this property is auto or «the given spine item is not orientation constrained,» meaning the element should render according to device orientation or default. (epubsecrets.com)
Reading Systems should render a Synthetic Spread for spine items only when the device is in portrait orientation. (idpf.org)
Reading Systems should render a Synthetic Spread for spine items only when the device is in landscape orientation. (idpf.org)