補足 and 訂正: adding missing information and correcting an error
Compare supplementing an incomplete explanation with a diagram and correcting errors in materials or a statement. Missing and incorrect information call for different responses.
Adding missing information
補足 (ほそく) means supplementing information. 「説明が足りない部分を図で補足した。」 says a diagram supplemented the part with insufficient explanation.
「担当者から日程について補足の説明がありました。」 says the coordinator supplied additional information about the schedule. Retain the schedule as topic and the coordinator as the source.
Correcting incorrect content
訂正 (ていせい) means correction. 「資料の誤字を見つけ、その場で訂正した。」 says a typo in the materials was found and corrected on the spot.
「先ほどの発言に誤りがあったので訂正します。」 announces a correction because the earlier statement contained an error. Its nonpast declaration does not report an already completed correction.
Responding to lack or error
When missing explanation leaves needed information absent, examine supplementation. When supplied content is wrong, examine correction of that content.
Supplementing information is not equivalent to attaching a file. Correcting an error is also different from withdrawing the entire statement; both supplementation and correction can be relevant when information is missing and mistaken.
What role does the diagram have in 「説明が足りない部分を図で補足した。」?
Show the answer and why
It adds needed information to an insufficient explanation. The sentence does not state that the original explanation contained an error or that a statement was withdrawn.