I have no idea what 'the spectrum' is.
Calibre IS the Library. Self contained. Metadata and books.
There is no need for Save to Disk. That is just
a choice when you go to EXPORT a book to a unsupported device.
Any linking is to enable coupling additional unsupported items into the Library.
There is no official Manual other than the users guide. MR members have come up with creative solutions and creative adaptations

Those are documented

in these forum