It is a common-known fact that end user is the most significant person in the project.
So to start proper testing software testers need a set of data form the end user.
The customer should also guarantee the exact data for software testing.
Using exact data at the time of testing is very significant. In many situations testers key in wrong data and expected results which are of no interest to the client.
Testing team also need the requirement document from the customer.
In general the customer never writes a formal document until he is really sure in his requirements.
The customer should also mark the risky areas of the project. For example, in a standard accounting project if a voucher entry screen doesn’t work that will break the functionality absolutely. But if reports are not derived the accounting department can use it for some time.
The customer is the right person to define which part of the project will have the biggest affect. Together with such feedback software testers can make ready a correct test plan for those parts and conduct testing entirely.