Meaning of "part 2 of the main quest" in the summer event for making Scáthach permanent?

The summer event is split into two parts, right now we can only do part 1 but within a few days or so, part 2 will be released. When part 2 is released we’ll probably have to complete the main story of it to get Scathach permanently. These kinds of events have nothing to do with the main story of Fate/GO, except for the requirements to do the event. Hope this helps.