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.
it also does more than just coordinate syncing across teams, machines, and environments. it also encrypts and holds your secrets to be injected just in time.
meaning you can use .env.vault files without dependence on a 3rd party. more on the Locally Managed.
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.