What do square brackets around a letter mean?

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Tags

1. #enclose
2. #record
3. #pronoun
4. #sentence
5. #employer
6. #espinoza
7. #change
8. #falsification
9. #brackets
10. #bracket
11. #quoting
12. #material
13. #former
14. #letter
15. #within
16. #changed
17. #capitalization
18. #coaching
19. #charged