A Reboot is a (graceful) Shutdown followed by a Start, much like rebooting a physical machine using the "shutdown and restart" option. It is commonly used to apply changed to the virtual hardware configuration (which Proxmox does in between the shutdown and start) without having to shutdown and wait for it to complete before starting it again. A Reset is very much like unplugging the power cord temporarily and turning it on again (or pressing the reset button on a physical machine) and does not allow for changes to the virtual hardware to be applied (much like you won't be able to applied physical changes in between pressing and releasing the reset button).