A Medium 9×9 Puzzle Solved with Forced Chain in 15 Steps

This 9×9 board rates as medium (difficulty score 915). Solving it from scratch takes 15 logical steps, using 7 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Line confinement, Paired regions, Forced chain. Every step below is forced: nothing here is a guess.

1. First, together, two regions have open cells only in columns A and B; with one marker each, those two markers have to fill exactly those two columns, so every other region's open cells there can be cleared.
2. Next, every open cell left in this region sits in column C, so its marker has to land there; that clears every other open cell in the column.
3. Then, every open cell left in this region sits in column D, so its marker has to land there; that clears every other open cell in the column.
4. After that, every open cell left in this region sits in column E, so its marker has to land there; that clears every other open cell in the column.
5. Now, every open cell in row 1 belongs to the same region, so that region's marker has to be somewhere in this row; that clears every other open cell in the region.
6. From there, place a marker at F2. Row 2 has exactly one open cell left. Placing here clears the rest of row 2, column F, and its region. Placing here also clears its 1 touching neighbour.
7. Following that, place a marker at C3. Row 3 has exactly one open cell left. Placing here clears the rest of row 3, column C, and its region. Placing here also clears its 1 touching neighbour.
8. First, place a marker at D5. Region has exactly one open cell left. Placing here clears the rest of row 5, column D, and its region. Placing here also clears its 1 touching neighbour.
9. Next, place a marker at I4. Row 4 has exactly one open cell left. Placing here clears the rest of row 4, column I, and its region.
10. Then, place a marker at H1. Region has exactly one open cell left. Placing here clears the rest of row 1, column H, and its region.
11. After that, if G6 were the marker, it would force A8 (the last open cell in row 8), and column B would end up with no open cell for its marker, which can't happen. So it can't go there.
12. Now, place a marker at A6. Row 6 has exactly one open cell left. Placing here clears the rest of row 6, column A, and its region.
13. From there, place a marker at B9. Region has exactly one open cell left. Placing here clears the rest of row 9, column B, and its region.
14. Following that, place a marker at E7. Region has exactly one open cell left. Placing here clears the rest of row 7, column E, and its region.
15. Finally, place a marker at G8. Region has exactly one open cell left.

The key move here was Forced chain. Once you can spot that, this board falls quickly. ∎