Based on the original BZID, a-id is an anonymous cryptographic secure identifier.
An anon-id is composed of two parts: an a-id (public, username, 16 characters) and an h-id (private, password, 32 characters).
Once an anon-id is created (python3 a-id.py --create < secret-key.txt) the a-id can be displayed and, only together with the h-id, can be validated (python3 a-id.py --verify anon-id < secret-key.txt).
As long as the h-id is not compromised, it is virtually impossible to validate an anon-id solely from the public a-id.
This enables each user to build a reputation within a website/app using their provided a-id, with the assurance that these anonymous identifiers are extremely difficult to forge.
One user may use multiple a-ids, but each a-id is used by one user only.
Examples of websites using this concept of anonymous identifiers on their leaderboards: Cross-Pollinator, AlgoDeduce.
Note that the actual a-id representation (image or avatar) can vary to fit the website/app graphic design.