You can think of it like a unique git url at GitHub. It identifies your project so that your teammates pull the correct .env file from dotenv-vault.
Here's an example of what a .env.vault file looks like:
The first thing you might notice is the formatting is the same as a .env file. This is intentional to allow for maximum future interoperability.
The DOTENV_VAULT key is important. It is required. It is that value that identifies your project uniquely to the dotenv vaulting mechanisms.