There are two USB ports on the E18.
The one labeled "Power" is strictly for charging the E18.
The one labeled "USB" can either play music from your source DAP or charge your source DAP as per the setting of the switch next to the USB port.
You can charge your E18 while playing music from either your source DAC or line input.
I was answering someone else's post on here. I fully understand the functions and ports. Thanks though