Is a functional spec a part of the System requirement spec?

Question

I wonder, sources like wikipedia or templates I found shows that Functional spec is a part of System requirement documents. I always thought that SRD is just overall decsription of the system, with all functional and non functional requirements. Yet I thought that Functional spec is more detailed and it is a separate document, while SRD is high level customer-created description (how is this one called then?) Could anyone help to make this clear for me?