What does the letter's impact mean?

[Impacty]

Business English Lesson, Advanced Level

ESL/EFL Test #139 "Business Letter Basics", question 10

Unfortunately, the ending paragraphs in much commercial correspondence employ wordy and overused phrases that detract from the letter's ..........

(a) contact
(b) contract
(c) extract
(d) impact

Business English Lesson, Advanced Level

ESL/EFL Test #139 "Business Letter Basics", answer 10

Unfortunately, the ending paragraphs in much commercial correspondence employ wordy and overused phrases that detract from the letter's impact.

Correct answer: (d) impact
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I think I understand the overall meaning, but this is a very complicated sentence. Is this a complete sentence? Where is the subject, verb? I believe that "the ending paragraphs with a lot of commercial correspondence that are wordy and overuse phrases distract the letters's impact." Is this right? But then why is there that before detract from again? Thanks for your help

[Beeesneees]

Subject:
The ending paragraphs

Verb:
employ
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Cheers m' dears!