In a BIP39 seed phrase, not every word can be used as a final word. The last word serves as a kind of checksum, ensuring that the seed phrase follows the rules laid out in the BIP39 standard.

In case of 24-word seedphrase, checksum is calculated by hashing the bits of first 23 words along with the first 3 bits of the 24th word, in order to find one of the valid checksum words.