German term
Unterbeleg
My inclination is to translate 'Beleg' as 'document' and 'Unterbelege' as 'Supporting documents'. However, 'Beleg' can be translated in many different ways and it is possible that 'Unterbelege' would be better translated some other way. The suggestions I have found, such as 'sub-voucher' and 'child document' just don't seem to cut it, at least in this context. I would therefore be very grateful for an authoritative suggestion.
| 3 +2 | supplementary record |
Johanna Timm, PhD
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| 3 | secondary receipt |
Adrian MM.
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Proposed translations
Canada
Friendly, fast & certified!
Native in: German (Variants: Swiss, Platt / Nieder (Low German), Austrian, Germany) 
Works in: English to German, German to English, German
supplementary record
Die GBV unterscheidet nicht zwischen Hauptbelegen, also Verträgen, Grundbuchanmeldungen usw., und Unterbelegen (teilweise auch als Nebenbelege o. ä . bezeichnet) Unterbelege sind „Beilagen“ zu den Hauptbelegen Ausweiskopien oder schriftliche Erklärungen nach Art 51[…] stellen Unterbelege dar.
https://fedlex.data.admin.ch/filestore/fedlex.data.admin.ch/...
Beleg = record
(could not access the full site, but it relates to the Swiss land register)
https://www.shareweb.ch/site/Agriculture-and-Food-Security/f...
| agree |
Barbara Schmidt, M.A. (X)
5 days
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| agree |
Marijke Verhagen (X)
102 days
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Thanks 😀
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Austria
Retd.T&I, Barrister & Austro-Advokat
Native in: English (Variants: British, Scottish) 
Works in: Spanish to English, English to Spanish, French to English, and 5 more.
secondary receipt
The Unterbelege could also refer to 'subsequent receipts for contract version updates' - unclear from the context.
Below-the-line receipt : has a different income-receiving meaning.
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Note added at 8 hrs (2021-10-14 19:12:25 GMT)
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In the context of a Swiss land registry, as intimated: sub-entry / sub-entries - https://www.sapdatasheet.org/abap/tabl/vilmemcn-secsubno.htm...
UK HM Land Registry: documents supporting rectification - alteration - of the register, if such is the case and adversely affects the regd. owner https://www.gov.uk/government/publications/rectification-and...
Für Unterbeleg & von Beleg & ist keine weitere Aufspaltung möglich Further itemization not possible for subdocument & of receipt
shippers are not entitled to use secondary downstream receipt and delivery points unless they contract with the pipeline for downstream capacity .
http://dev-workbench.com/de/sap-dictionary/message-class/ptra_web_interface
http://dictionary.cambridge.org/dictionary/english/below-the-line
Discussion
Now that I have spent time working on the document, it has become apparent that the agreement relates to changes in the land registry. (It actually originates from a land registry in a Swiss town.) With this in view, I think the specific context might possibly be better served with 'Entry' for "Beleg' and 'Supporting entries' for 'Unterbelege'.
I would therefore appreciate any comments relating to whether this is the best interpretation in view of this narrower context.