I was looking at an ICAO flight plan and observed that there are only specific ITEMS mentioned in it (see example below)? What is the reason behind excluding the item numbers 1, 2, 3, 4, 5, 6, 11, 12, 14 and 17?
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To complement @ocirocir 's answer and link, the messages used by air traffic services are not limited to flight plans.
Below from ICAO Doc 4444 are all the fields and the intended use. As you see, the FPL is just one row, and other "missing" fields appear in other messages, while others are just currently not used: