The dash_cloud example is more of a code-walkthrough than anything. The Serial USB is not used in that sketch, so that’s why you don’t see it. The main message send functions are demonstrated in that sketch, along with sleep states. In deep sleep, the USB connection is terminated, so the serial port would be closed most of the time anyway.
When the Dash(Pro) is put into program mode, the Serial USB port is closed, so the port ‘goes away’. To get the Serial USB port to open in your sketch, call Serial.begin(); in the setup() function. The OS is in charge of assigning the actual port designation, so it may change when the Dash(Pro) is reset or re-programmed.
The rules file in Ubuntu designates the Dash(Pro) Serial USB port as a ‘normal’ serial port. Without the rules file, Ubuntu would treat the Dash(Pro) like a modem and try to send it AT commands, which doesn’t work for the normal Dash(Pro) setup.
If you want to try out the Dash(Pro) features, program the dash_repl_basic sketch from the Examples in the included DashReadEvalPrint library. It is also available on our Github repo dash_repl_basic.ino. This gives you a command prompt that allows you to control the Dash(Pro) via shell style commands over USB Serial. Press enter after loading to get a prompt. Type help to see a list of commands. For example:
cloud send "Hello, World!"
sends the message Hello, World! to the cloud.
Those are all great questions. Post more if you have them.