Q.What are "PC site" and "Mobile site" text boxes and options?

Throughout the Shop Manager, you may see separate text boxes or options for PC and mobile sites:

If so, this means you have a PC-only template set at Design → Design templates. If you have a PC-only template set, then you have a separate mobile website that appears when accessed via a mobile device such as a phone. These options allow you to set different text on your main (PC) site and your mobile site.

When using a Responsive template, the website is the same and just viewed in a different format, and these fields are not separate: