Eurocontrol says that Training Flight must be filed as "G", and in field 18 RMK/TRAINING FLIGHT FOR THE PURPOSE OF OBTAINING A LICENSE" in order to be exempted from route charges (for domestic flights)
Interesting! I found that information on the Eurocontrol website too, but this must be one of the areas where national regulations overrule ICAO and Eurocontrol.
Because in Germany a very recent aic (in german & english language:
http://www.dfs.de/dfs/internet_2008/...c_ifr_2_12.pdf ) states that in order to pay reduced fees for training flights, an "X" is to be entered and "RMK/LIC" in field 18... (and that's how we've been doin it as long as I can remember.)