A 'libris' datastore is a directory containing subdirectories named after the various libris formats, each of which containing libris files of the same format.
A resource in the 'libris' datastore is a data inside a libri file. Most libris have 1 data per file, but lhz (for horizons) can contain multiple horizons. That is why, generally speaking, a "libris://" URI designates a specific data inside a specific file. Example:
/home/joe/data/libris
|-- lfd
| |-- foo.lfd
| `-- bar.lfd
|-- lgr
| `-- quux.lgr
|-- lhz
| `-- foo.lhz
|-- lmu
| |-- foo.lmu
| |-- bar.lmu
| `-- quux.lmu
`-- lvi
|-- foo.lvi
|-- bar.lvi
`-- quux.lvi
Here, the datastore root directory is /home/joe/data/libris.

In fact, files can be left anywhere, even in the root directory. But the files you will write will be automatically saved in the subdirectories.

The foo.lhz file can contain multiple horizons: H1, H2, H3... The URI for an horizon will specify the horizon's name.