The Pleno al 15 is the special category of the Spanish Quiniela: you have to predict the exact score (number of goals for each team) of the fifteenth match on the ticket. It was introduced in 1988 and only counts as a prize if you've also gotten the 14 previous 1X2 predictions right.
Spain's Quiniela was born in 1946, with a first round of 7 matches that grew over the decades to the 14 matches typically played today, all using a simple 1X2 pick: 1 (home win), X (draw), or 2 (away win). The Pleno al 15 wasn't there from the start: it was added in 1988 as an extra category, designed to boost the ticket's appeal by adding a fifteenth match with a different mechanic from the first 14. Since then it's been an inseparable part of the Quiniela as we know it today.
Unlike the first 14 matches, the Pleno al 15 doesn't ask who wins — it asks for the exact score in goals for both teams, within a closed set of combinations: 0-0, 1-0, 0-1, 2-0, 0-2, 2-1, 1-2, M-0, 0-M, or M-M, where "M" means "more than two goals". This makes it far harder than a simple 1X2: it's not enough to pick the winner, you have to nail the actual scoreline within those categories.
The most important nuance, and the one that confuses most people filling in a Quiniela for the first time, is that the Pleno al 15 isn't an independent prize: it only activates if the ticket has already gotten all 14 previous predictions right. If you miss even one of the first 14 matches, it doesn't matter if you nail the fifteenth match's exact score: that hit counts for nothing. In practice, it's a tiebreaker and a prize multiplier for those who've already gone 14-for-14, not a standalone route to a prize. Financially, 7.5% of each round's total takings is set aside for the Pleno al 15, plus any accumulated jackpot from previous rounds with no winners in that category, which explains why it sometimes generates prizes far bigger than the other categories.
The spirit of the Pleno al 15 — rewarding the exact-score caller more than just the winner — is the same principle Match Prophets uses in its default scoring system: 3 points if you nail a match's exact result, 2 if you just get the winner or draw right. The difference is that in your pool nothing else needs to line up for it to count, and there's no real money involved: just points and your group's standings.