Oftentimes, organizations use outside vendors or subcontractors as part of their business processes. Some of these vendors need access to data in order to perform the function for which they are contracted. As a result, some risks that the vendor has are now assumed by your organization since they have access to the data you are charged with protecting.
The addition of vendors or subcontractors to the list of people with access to an organization’s data, even if it through a partner organization, increases risk. They want to know if there are other organizations besides yours that will have access to the data so that they can understand the risk of sharing the data with you.
They are asking if you share the data with any of your vendors or subcontractors. They may ask for assurances that your vendors and subcontractors uphold the same security standard as they expect you to uphold. It is also possible that they will want a list of those organizations so that they can take the same due diligence with them as they are taking with you