USB port selection criteria during i.mx8MP serial boot download (details)

[nicolaponzevero]

I read the post at https://community.nxp.com/t5/i-MX-Processors/select-usb-port/m-p/1632067#M204074 about the USB port selection for the USB serial download in i.MX8MP

It mentions Section 6.1.7 USB boot of i.MX 8M Plus Applications Processor Reference Manual. that starts with:

ROM supports both of the USB ports on the chip for boot purposes. Only one port can be selected as the boot connection. The port that is active first wins the selection.  

I have a few additional questions:

1. Can you clarify what "active" means in this context?
   1. Is the connection that receives the 1st byte (URB) after the poweron selected?
   2. are there other conditions making a port "active"? e.g.
      1. events like connect something to the port
      2. a current flowing in the power lines
      3. ...
2. Is there any application note providing guidelines for designing a circuit allowing the use of an USB port for reliably supporting the serial download?    
   1. Can a port connected to an HUB or to a device result "active" for some reason?
   2. Can the low level USB traffic (enumeration) affect the detection?

Thanks and best regards

N.