Logic Masters Deutschland e.V.

X-Sums: A circled number outside the grid is the sum of the first X digits looking in from that position, where X is the first digit encountered.

Progressive Divisors: The X-digit number forming the X-sum is a progressive divisor number - for each N up to X, the number given by the first N digits is divisible by N. (So, for instance, 4896 would be a valid fill for a 27 clue, since 4 is divisible by 1, 48 is divisible by 2, 489 is divisible by 3, 4896 is divisible by 4, and the digits sum to 27.)

f-puzzles: https://f-puzzles.com/?id=y3xljedo
CTC: https://tinyurl.com/yeyr53ap