Reduced Vertical Separation Minimum (RVSM)
The NextGen flight planning engine takes into account equipment code "W" (RVSM) in field 10a of the ICAO flight plan. Without this code, your flight plan will be held below FL290 in order to meet this regulatory requirement. Please note that you will also not be able to generate a plan above RVSM airspace (altitudes higher than FL410) even if the aircraft’s performance is capable of reaching those flight levels.
European Data Link Mandate
Now that increased travel to and within European airspace is on the horizon, it is a great time to revisit this regulation. The European Data Link Mandate went into effect in February 2020 when much of the world was starting to grapple with the impact of COVID-19. The mandate requires that airspace users operating above FL285 be equipped for ATN/Link2000 CPDLC communications (“J1” in field 10a of the ICAO flight plan), unless exempt. Most General Aviation aircraft are exempt from the mandate by way of the following from the European Commission: "Aircraft with a maximum seating capacity of 19 or less and a Maximum Take-Off Mass (MTOM) of 100,000lbs or less with an airworthiness certificate dating before 05 February 2020."
Additional permanent exemptions for airframes, regardless of the airworthiness issue date, can be found here within the Annex C document under the Commission adoption header. Aircraft that are exempt will need to include "Z" in field 10a of the ICAO flight plan in addition to CPDLCX in the DAT/ section. Without the J1 or CPDLCX codes, the flight planning engine will cap the flight level of your plan to FL285 and below.