A Project key is a unique identifier of your Project. Yodiz will automatically generate a short project key based on your project name. However, if you wish to designate a short project name yourself, you can do so. On certain views like To-Do and Reports, where data is shown from multiple projects, this key helps users to identify the contents and their relative projects. External users also need this key, if they want to create work items in Yodiz using Interact Via Email.