A "problem" refers to the process of generating a full Sudoku solution or a Sudoku puzzle with holes. An "instance" is when we are checking sat for a particular index when generating the whole sudoku "problem".
smt2_files: store problems/instances in SMT format
txt_files: store problems/instances in txt format
./particular_hard_instances_records stores particular classic/argyle instance that are hard in smt and txt format
./whole_problem_records stores whole classic/argyle problems that are hard
The smt2_files and txt_files subdirectories under these directories store problems/instances in SMT and txt format