Introduction
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Aster commands exchange data. Most often these data are Data Structures (or "user" concepts) for which, the programmer does not have to do "READ" (for his data) or "WRITE" (for his results). In this case, the "inputs"/" exits" are done by JEVEUX [:external:ref:`D6.02.01 <D6.02.01>`].

However, sometimes some commands (usually procedures) have to read data from a file or write a result. The rules concerning these orders are given in § :ref:`4 <RefHeading__29464803>`.

During a calculation, a command may want to emit an error or alarm message, in which case it will use the "package" UTMESS [D6.04.01]. We'll talk about it again in § :ref:`5 <RefHeading__29474811>`.

Finally, a command may want to write information messages concerning the progress of the calculation. These impressions are governed by the order's INFO keyword. These prints are the subject of § :ref:`3.5 <RefHeading__29494966>` for which the INFXXX [:external:ref:`D6.04.02 <D6.04.02>`] package is used).