How many clubs a country sends to European competition, and where they enter, is determined by a coefficient calculated from years of previous results. The ranking is settled long before the current season begins.
Two coefficients do different jobs
A country coefficient aggregates the performance of all a nation's clubs across several seasons and governs how many entries that nation receives.
A club coefficient tracks an individual team's own European record and is used for seeding within the draw.
Both are calculated over multi-season windows, so a single strong or weak campaign moves them slowly and the rankings are effectively lagging indicators.
Entry places are allocated years in advance
Because the coefficient window closes before the qualifying rounds are drawn, the number of places a country receives reflects results from earlier seasons entirely.
A league improving rapidly therefore sends fewer clubs than its current strength would justify, and a declining one sends more.
The lag is deliberate, since allocations must be fixed before domestic seasons finish, but it means the format is always describing the recent past.
Why deep runs by weaker clubs matter so much
A country's coefficient is averaged across the clubs it enters, so a small club progressing further than expected raises the national figure disproportionately.
Early elimination of several entrants drags the average down even if the strongest club goes deep, because the calculation divides by the number of teams entered.
This is why supporters of rival domestic clubs occasionally find themselves willing each other on in Europe.
Performance places reward the whole league
Additional entries are now awarded to the countries whose clubs performed best across the previous season, which converts collective European performance into a concrete extra place.
The place goes to the highest-finishing domestic club that has not otherwise qualified, so it changes the value of positions in the middle of a table.
Domestic races are therefore affected by results in matches those clubs had no part in.
What seeding actually buys
A higher club coefficient produces a more favourable set of opponents in the draw, and in a league-phase format it shapes the whole schedule rather than one tie.
Historically strong clubs carry high coefficients for years after their form declines, which is how a struggling side can still be seeded generously.
The system rewards sustained success and punishes volatility, which is a defensible design and a poor description of current strength.