A group stage draw is not a free allocation of clubs to groups. Teams are sorted into pots first, and the pots do most of the work of balancing the competition.
Pots distribute strength evenly across groups
Clubs are ranked and divided into bands of equal size. Each group receives exactly one club from each band, so no group can contain several of the strongest entrants.
Without pots, random allocation would regularly produce one group containing three leading clubs and another containing none, which distorts the competition before a match is played.
The design guarantees a spread of difficulty rather than an equal difficulty, since the specific clubs drawn still vary considerably in strength within each band.
Domestic champions are treated separately
Some competitions place national champions in the top pot regardless of coefficient, which prioritises the sporting achievement of winning a league over accumulated continental history.
This displaces clubs with stronger coefficients into lower pots, and those clubs frequently end up in harder groups as a result.
The rule is a value judgement rather than a technical one, and it is periodically revisited because it produces exactly that friction.
Constraints limit which combinations are legal
Clubs from the same country are usually prevented from meeting in a group, which is intended to preserve variety and to avoid domestic rivalries deciding continental qualification.
Additional restrictions cover clubs whose home matches cannot be scheduled in the same week for policing reasons, and occasionally political constraints between countries.
These conditions interact, and by the final pot the constraints can leave only one legal option, which is why draws are increasingly assisted by software.
Constraints change the probabilities in ways that are not obvious
Because a club cannot be drawn against certain opponents, the remaining possibilities are redistributed among the rest, and those probabilities are not uniform.
A club from a country with many entrants faces a different distribution of possible opponents than one from a country with a single representative.
The effect is small in most cases but real, and it means describing a draw as random without qualification is inaccurate.
League-phase formats replace groups with a ranked schedule
Formats that place all clubs in a single table still use pots, but to determine opponents rather than groups, giving each club a set number of fixtures against clubs from each band.
This produces an individual schedule for every club, so no two teams face the same opposition and strength of schedule becomes a genuine variable.
Balancing that requires the pot structure to be tighter, since the draw is now distributing difficulty across many separate fixture lists rather than across a handful of groups.
