should be general (independent, e.g. specific individuals carrying out the activity), but should clearly specify the entity. Processes should be numbered for easier mapping and referral to specific processes. The numbering is random, however, it is necessary to maintain consistency across all DFD levels (see DFD Hierarchy). DFD should be clear, as the maximum number of processes in one DFD is recommended to be from 6 to 9, minimum is 3 processes in one DFD. The exception is the so-called contextual diagram where the only process symbolizes the model system and all terminators with which the system communicates. DFD consistency DFD must be consistent with other models of the system - entity relationship diagram, state-transition diagram, data dictionary, and process specification models. Each process must have its name, inputs and outputs. Each flow should have its name (exception see Flow). Each Data store must have input and output flow. Input and output flows do not have to be displayed in one DFD - but they must exist in another DFD describing the same system. An exception is warehouse standing outside the system (external storage) with which the system communicates. DFD hierarchy To make the DFD more transparent (i.e. not too many processes), multi-level DFDs can be created. DFDs that are at a higher level are less detailed (aggregate more detailed DFD at lower levels). The contextual DFD is the highest in the hierarchy (see DFD Creation Rules). The so-called zero level is followed by DFD 0, starting with process numbering (e.g., process 1, process 2). In the next, the so-called first level - DFD 1 - the numbering continues. E.g. process 1 is divided into the first three levels of the DFD, which are numbered 1.1, 1.2 and 1.3. Similarly, processes in the second level (DFD 2) are numbered eg 2.1.1, 2.1.2, 2.1.3 and 2.1.4. The number of levels depends on the size of the model system. DFD 0 processes may not have the same number of decomposition levels. DFD 0 contains the most important (aggregated) system functions. The lowest level should include processes that make it possible to create a process specification for roughly one A4 page. If the mini-specification should be longer, it is appropriate to create an additional level for the process where it will be decomposed into multiple processes. For a clear overview of the entire DFD hierarchy, a vertical (cross-sectional) diagram can be created. The