This is a 6×6 sized puzzle, so
- Every row must contain the numbers 1 to 6 once.
- Every column must contain the numbers 1 to 6 once.
- Each chain of cells (joined by a line) contains 6 cells, and must contain the numbers 1 to 6 once.
If you are looking at a Chainlink puzzle that is 7×7 in size, then every row/column/chain would need to
contain the numbers 1 to 7 once.
These rules are very similar to Sudoku, and this puzzle is sometimes called ChainLink Sudoku.