In this list, each word is uniquely identified by its first four letters. “Unique” here refers to the exact first four letters of each word, not the first four distinct letters. For instance, “appl” from “apple” is distinct and does not appear as the first four letters of any other word in the BIP39 wordlist.
For words composed of only three letters, such as “add,” there is no fourth letter. Therefore, “add” stands alone. Although “addict” is also in the BIP39 wordlist, it would be distinguished by the first four letters “addi.” This ensures that no two words in the list share the same initial four characters.

Thus, if you input the first four letters of a word, you can determine the complete word by referencing these letters in the BIP39 wordlist. Some wallets may automatically complete the word once the first four letters are input. According to the BIP 39 documentation, this principle is clearly stated:

• https://github.com/bitcoin/bips/blob/master/bip-0039/bip-0039-wordlists.md
• https://github.com/bitcoin/bips/blob/master/bip-0039/bip-0039-wordlists