UEFA Champions League draw rules state that:
- A team is drawn by a human (generally a famous player).
- The computer indicates the possible groups in which that team can be drawn.
- A human puts the corresponding balls of possible groups into a separate pot.
- A group is drawn from that separate pot by a human (another famous player).
There are 4 pots with the names of the teams allocated to these pots by their rankings and additional 8 pots each containing only the group letters (i.e, a pot with only “A”s, a pot with only “B”s and so up to a pot with only “H”s). Externally the balls that contain team or group names should look completely identical.
- Firstly, teams from pot 1 are drawn (one team is drawn per each group without constraints).
- For the draw with teams in the 2nd/3rd/4th pots there will be some constraints. So, after a team is drawn, the computer suggests in which groups that team may be allocated.
- Then an attendant creates a separate pot with the only group-letters where the team can be drawn into. This operation is made after the draw of each team based on the most current constraints.
- Finally, a group is drawn for the selected team from that separate pot.
This is the youtube video of 2015 Champions League draw.
Here you can see that Real Madrid is drawn. Computer shows that only groups A and D are available for Real Madrid and then the responsible takes one ball from pot A and another from pot D and puts them into a separate pot, from where the final post for Real Madrid is drawn.
At some point it’s also possible that a team can be drawn only into one group (Valencia can be allocated only into group H), so there is no need to draw the group ball.