Dear Gabriel,
thanks for your answers anyway. Negative postings have multiple use in accounting, therefore you have this tick-box when posting an FI document:
Yes, it is usually a reversal, but not always. There are certain corrections that may (or even should) be done in this way. But this can be country-specific...In my cases they are reversals but the bank does not provide the link to the original transaction, therefore I cannot use a reversal, only a simple GL posting. I will keep digging 
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Regards,
Ondrej