Recommended with acknowledgment of receipt

How to collect a registered letter with acknowledgment of receipt?
When you receive a registered letter with acknowledgment of receipt, two scenarios arise.

At the time of the postman's delivery, the recipient is present
The postman will then hand over the correspondence after having the recipient sign the acknowledgment of receipt for return to the sender,

At the time of the postman's delivery, the recipient is absent
The postman will then leave a notice of passage in the mailbox. To collect the letter, it will be necessary to go to the address of the post office, usually the closest one, indicated on the notice of passage, carrying an ID and the notice of passage to retrieve the correspondence at the counter (with the acknowledgment of receipt signed on site).

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I would like to know what the purpose of sending a registered letter is.

Recommended means that the letter posted on day X is received the next day. It is actually used to send a letter more quickly than usual.
For the acknowledgment of receipt, the sender receives a paper that shows that he has indeed sent the letter and that it has arrived safely.

It's just that they sent it by registered mail to make sure you receive it and have a receipt that proves you got it.
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I didn't understand everything, please help me.

Hi
Actually, the sender wants proof that you received what they sent you.
Check out this article
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So I need to keep the letter?

And should I send this same letter?

Yes, but you have to return the acknowledgment of receipt
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