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The reason is primarily related to how NOTAM are currently selected for inclusion in Pre-flight Information Bulletins (PIB), based on the item A (airport code or FIR code). It has to be assured that the NOTAM appear correctly in the relevant en-route and airport PIB and that duplication in PIB (e.g. in the FIR) are avoided. Depending on the number of affected airports and FIR, NOTAM format limitations such as the limitation of Item A (maximum 7 FIR or 1 airport), NOTAM series allocation or other national requirements, different options exists to assure correct PIB inclusion. The NOTAM for the en-route and airport part of the PIB may also be combined, provided NOTAM rules are respected. This is often done to reduce the number of NOTAM issued.
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