The 48 national teams were divided into ten groups (two groups of four + eight groups of 5). Each group winner, as well as the six highest ranked second placed teams, advanced to the play-off. One of the eight qualifiers was then chosen to host the remaining fixtures.
Ranking of second-placed teams
Because some groups contained five teams and some four, matches against the fifth-placed team in each group are not included in the ranking. As a result, six matches played by each team counted for the purposes of the second-placed table. The top six advanced to the play-off.
References
2004 UEFA European Under-21 Championship qualification Wikipedia(Text) CC BY-SA