In a split diff view, the side of the diff that the pull request’s changes appear on. Can be LEFT or RIGHT. Use LEFT for deletions that appear in red. Use RIGHT for additions that appear in green or unchanged lines that appear in white and are shown for context. For a multi-line comment, side represents whether the last line of the comment range is a deletion or addition. For more information, see "Diff view options" in the GitHub Help documentation.
Required when using multi-line comments unless using in_reply_to. The start_side is the starting side of the diff that the comment applies to. Can be LEFT or RIGHT. To learn more about multi-line comments, see "Commenting on a pull request" in the GitHub Help documentation. See side in this table for additional context.