The Commodore 64 Ultimate has two type of specifications, the real hardware specifications and the programmed specifications.
The 'Commodore 64' part is powered by FPGA hardware.
Fundamentally, an FPGA‑based Commodore 64 is a hardware recreation of the original computer.
The FPGA is programmed with the exact logic of the C64’s CPU, video, sound, and I/O chips, so it behaves just like the 1982 hardware—even though the underlying chip is far more powerful.
For the user, the experience is indistinguishable from using a real C64: the same keyboard, ports, video output, and timing, allowing original software to run unchanged.
The being said, the actual hardware has more resources then an original C64 ever had available.
Because of that, the C64U has configuration options that the original did not have, 'Comfort of life' options. You can easily switch between PAL or NTSC video output, and have the ability to simulate original hardware, like a Commodore datasette or floppy drive, allocate more RAM or even increase the clock speed.
You are not going to run Windows or something like this on the C64U 1) but it can behave as an extremely souped up original Commodore 64.
Although a stereo audio jack may physically accept a 3.5 mm headphone plug, using it this way is not recommended and may potentially damage the device. It is advised to treat this connector strictly as a line‑out and use a headphone amplifier between the device and your headphones.
DB-9 joystick/paddle ports not all devices that have a DB-9 connector and work fine are advised to be used. Some devices are wired in a way that can damage your C64.