Excerpt from EPUB 3 Publications section 4.3.2 (strong emphasize mine):

Cardinality: In the metadata section: zero or more; Attached to other metadata: zero or one

Other metadata? Outside of <metadata>?!? I don't understand. At all.

It can't be related to the META-INF/metadata.xml file since section 2.5.4 of EPUB3 Container Format reads (strong emphasize mine):

This file, if present, must be used for container-level metadata. This version of the OCF specification does not specify any container-level metadata.

And the other sections of the OPF file do not contain metadata but a manifest, a spine and other stuff but no metadata...

I think - but I can't be sure - it in fact means:

Cardinality: as a primary expression (i.e. applies to the Publication): zero or more; as a subexpression (i.e. refining a primary expression or another subexpression: zero or one for the refined expression or subexpression

Anyway, the original prose is very badly worded, to say the least. Carefully reading the spec, I have no idea what are "other metadata" so there is at least an important definition (sic) here that remains undefined...