D1 Edexcel A level question

Hey everyone, I have my D1 exam next Thursday and I'm confused about something in the Matchings topic.


In part (d) in the mark scheme they connect E to 4, which is fair enough because they add E and 4 as a connection after part (b), but sometimes they connect them without two vertices being connected in the 'possible allocations' matching - I can't find an example of this exactly right now but I hope you understand what I mean. One of the rules is that you cannot connect something that isn't on the first matching, but sometimes they do, when the vertex only has one possible allocation.


Can someone please explain to me under what circumstances you can do that?