The system as currently documented is termed 'physical', since it is how the system appears in the real world. However, if we are to document what the system is actually doing we have to convert it into what we call its 'logical' form.
This is necessary because the physical system consists of features to enable the user to interact easily with it, but these features make it difficult to implement on the computer system.
To carry out logicalisation we apply a set of rules or procedures to our current physical system. These rules are applied to the current physical level 2 Data Flow Diagrams. The rules are shown below.