What is the DSR memory model at the heart of FSRS?

Three per-card variables: Difficulty, Stability, and Retrievability.

• Difficulty (D) — the card's inherent complexity, roughly on a 1–10 scale. Higher D means stability grows more slowly.
• Stability (S) — the number of days for recall probability to fall to 90%. Big S = a durable memory that decays slowly.
• Retrievability (R) — the probability you could recall the card right now, which decays with time since the last review.

Crucially, R changes every day (time passes), while D and S only change when you review. Separating "how durable" (S) from "how hard" (D) is exactly what SM-2's single ease factor could not do.