what is the meaning of the second + in the save file?
Solution 1:
According to this IGN article, the second number is the number of solved puzzles that do not involve panels.
Here's the number that a 100% game file should have, as far as current players are aware of:
523 +135 +6
The first number represents puzzles solved, the second number represents environmental puzzles completed, and the third number represents obelisks completed by finding all the associated environmental puzzles.