There are multiple definitions - click on the translation that matches your context.

Vertragsergänzung

Substantiv

partial

modification

noun

Comparative law notes

Austria

Since an addendum or supplement to a contract does in fact modify the contract, the term "modification" or "contract modification" is certainly not incorrect,

Definitions of modification

English

a small change to something, often in order to improve it

The court rejected a borrower's claim that there was an oral modification of his loan agreement.