There are multiple definitions - click on the translation that matches your context.
Untervollmacht means:
Substantiv
partial
noun
A sub-agency can also mean the appointment of a sub-agent who is delegated certain tasks for perform for the agent who had been authorized by the principal or client. In the US, it happens a lot in the real estate market among brokers.,
a small agency (= a business or organisation that provides a particular service on behalf of another business, person, or group) that is part of a larger agency
It has become a sub-agency of OgilvyOne.