Dataflows that carry a whole record between a datastore and a process is not labeled in the textbook since there is no ambiguity. This is also not a universal convention. I would rather you labeled such dataflows explicitly. What comes out of a datastore therefore must first have got into a datastore through a process. Processes can not create data out of thin air.
Processes can only manipulate data they have received from dataflows.
Data outflows from processes therefore must be derivable from the data inflows into such processes. The processes in the top level DFD are labelled consecutively by natural numbers beginning with 1. Datastores may be local to a specific level in the set of DFDs. A datastore is used only if it is referenced by more than one process. The context diagram must therefore show all external entities with which the system under study interacts.