You've learned DP on explicit tree structures. But what if the "tree" is implicit, defined by the problem's structure? Digit DP treats a number's decimal representation as a kind of tree: each digit is a node, and valid numbers are paths.
The constraint is usually "count numbers \leq N with property X". The DP state tracks position and whether we're still at the limit. Make sure you're comfortable with tree DP before moving on. Digit DP uses similar ideas but in a less visual context.