Medium Killer Sudoku gives you fewer starting digits, so the cage sums become real constraints rather than hints. Two-cell cages with unique combinations, the 45 rule for a full row or box, and simple innies and outies are enough to finish — but you will need all of them.
This is the default level when you open Killer Sudoku without a saved game. It suits players comfortable with classic Sudoku who want a puzzle that adds up. Compare it with medium classic Sudoku to feel the difference the cages make.