The meaning of 'assert subtask in ('all', 's2w', 'w2s', 'o2n', 'n2o', 'd2n', 'n2d')' in msls.py

[shenhai911]

Dear authors,
thank you very much for you extraordinary contributions. Could you please tell me the meaning of the code marked with red box in the picture below? (msls.py)
To be exact, what do 'all', 's2w', 'w2s', 'o2n', 'n2o', 'd2n' and 'n2d' stand for respectively?
Thank you!