The short answer
Automatic supports consistency; manual keeps the system simpler
A vacuum erection device uses a cylinder and pump to lower pressure around the penis, drawing blood into erectile tissue. With a manual model, the user creates each pressure change by squeezing a bulb, moving a handle or operating a lever. With an automatic model, a motor performs that work.
Automatic does not automatically mean smart. A smart pump adds a two-way app connection or other connected functions. A screen may display information without controlling the device, and a motor may run without measuring pressure. Check the documented features instead of relying on the category label. Power source is also separate from pumping medium; compare that decision in the air vs water penis pump guide.