[KevinH]

Found it here: https://www.w3.org/publishing/epub3/epub-packages.html

C.2.11 source-of

Name: source-of
Description:
The source-of property indicates a unique aspect of an adapted source resource that has been retained in the given Rendition of the EPUB Publication.
This specification defines the pagination value to indicate that the referenced dc:source element is the source of the pagebreak properties defined in the content.

Allowed value(s): pagination
Cardinality: zero or one
Extends: dc:source
NOTE
See [EPUBAccessibilityTechniques] for information on how to provide accessible page navigation.
EXAMPLE 51
The following example shows the ISBN identifier for an EPUB Publication together with the source ISBN identifier for the print work it was derived from.

<metadata xmlns:dc="http://purl.org/dc/elements/1.1/">
…
<dc:identifier id="isbn-id">urn:isbn:9780101010101</dc:identifier>
<meta refines="#isbn-id" property="identifier-type" scheme="onix:codelist5">15</meta>

<dc:source id="src-id">urn:isbn:9780375704024</dc:source>
<meta refines="#src-id" property="identifier-type" scheme="onix:codelist5">15</meta>
<meta refines="#src-id" property="source-of">pagination</meta>
…
</metadata>