This is used to uniquely identify the project within the organization. It can contain up to 10 alphanumeric characters and will be automatically generated from the project name.
The advantage of having a project identifier is that we can have shorter project URLs.
E.g. https://spinkart.neetotestify.com/BAX, here BAX is the project identifier.
The project identifier is also used for nomenclature and will be prefixed to any test cases created within the project.
In order to edit the identifier after project creation, you can go to Project Settings > Edit basic info and edit the identifier as per your needs.