Universal Serial Bus (USB)

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Nearly all computers and other portable devices now support a single Universal Serial Bus (USB) for both data and power.

Legacy serial ports on computers were slow speed and specific to a particular function, like: keyboard, mouse, audio in, audio out, serial RS232 port or printer port.
Now when a device is connected to a computer USB port is must identify itself so that the computer can recognize that device and ensure it has the correct device driver installed.
The smart card had a similar serial port with similar functionality. Since there were multiple card types, the card will "Answer to Reset (ATR)" with a code identifying it.^[1]
New Security Tokens for User Identifiers and Attributes are built to connect directly to the USB port and provide similar locator codes.

Since the first Smart Card was issued, portable identification devices have needed to issue Locator Codes of some sort which allow the attachment to acquire a software driver to support the card.
Smart cards have worked well for controlled environments like: governments, colleges and corporations. Consumers have never been willing to tolerate the complexity of the card and the X.509 Certificate they needed.
The first attempt was to add a card reader to convert from Smart Card format to USB, but the added hardware was enough to impede consumer adoption.
Even if the USB device worked, it was not recognized by the User Agent (browser) from companies like Microsoft, Apple and Google.

The first solution was a browser add-on from Google that would allow security keys that were plugged into the computer to be queried by the browser and perform a function just like "Answer to Reset".
A similar solution for Smart Phones include NFC and Bluetooth, explained elsewhere, none of which provide a secure physical connection.
An alternate solution for devices with Trusted Execution Environments is to place the User Identifier and Attributes into a well secured location within the device itself.