Explain customers/data_request GDPR webhook

[SaloEater]

Hi, I'm developing a Shopify application and now stuck with GDPR webhooks especially customers/data_request.
Is there any explanation what Shopify expects from us?
1) >The webhook contains the resource IDs of the customer data that you need to provide to the store owner

How am I expected to provide it?

2) >In some cases, a customer record contains only the customer's email address.

Does it mean request have dynamic fields volume?