[1] It refers to the immunity of user applications to changes made in the definition and organization of data.
Application programs should not, ideally, be exposed to details of data representation and storage.
Physical data independence deals with hiding the details of the storage structure from user applications.
The application should not be involved with these issues since, conceptually, there is no difference in the operations carried out against the data.
The logical scheme stays unchanged even though the storage space or type of some data is changed for reasons of optimization or reorganization.