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This procedure allows to store the value in a format or type that is different from the way the same is perceived in AggreGate.
Here is a simple example: Supposing, there is a setting that stores device temperature. The setting data originates from a temperature measuring IC, which outputs a byte value. Seven most significant bits are degrees. The least significant bit represent a 0.5-degree step. The output of 0 corresponds to -40 degrees C. Therefore, the output of 255 corresponds to +87.5 degrees C.
Now, the operator in AggreGare would surely prefer to see "natural" temperature values like -40 and +87.5, rather than 0 and 255. This procedure can provide a place to do the conversion.
The same possibility exists for stored event's fields (see Callback_agg_convert_event_field()) but not for table A-variables. Tables must be stored in exactly the same way that the AggreGate server "believes" them to be stored. This is because table synchronization is based on hash strings.