accounting and bookkeping services threat

What does the following paragraph relating to accounting and bookkeping services mean exactly?

Managerial decisions must not be made by the firm and the source data and subsequent adjustments must be originated or approved by the client.

I understood the part when it says that managerial decision cannot be made by the audit firm but I am not too sure about the meaning of the second part of the paragraph above starting with ”the source data”

thanks for your help.