What do circles mean in NYT Crossword?

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Tags

1. #letters
2. #related
3. #theme
4. #other
5. #circle
6. #squares
7. #contain
8. #white
9. #which
10. #consecutively
11. #spell
12. #rearranged
13. #words
14. #puzzle
15. #sometimes
16. #appear
17. #cheat
18. #square
19. #crossword
20. #either
21. #circles