hierarchical database model – physical pointers
A physical pointer is any value that indicates where something is located, e.g. a memory address, position in an array or collection, a file system path, OID, etc.
In contrast, the relational model identifies things by their own attributes – natural keys or assigned surrogate keys. This adds a layer of abstraction and complexity to the DBMS, but isolates users from the organization of system data structures.
For example, in hierarchical data structures, each child record might contain the address of its parent record, while a parent record might hold an array of its childrens addresses.