Well, as stated on page 7 of the Europa Plus instruction manual, socket 1 is the 'Main Earth Bond/Insulation socket', and would be used for Class I earth bond tests (apart from lead/extension earth bond tests), and Class II insulation resistance tests.
Socket 2 is called the 'Aux Earth Bond Connector', but unfortunately the manual then completely fails to mention it again.
It does have a symbol next to it that looks a bit like a clamp meter, but that's just a guess.
Maybe someone from Seaward might be able to enlighten us as to its usage, seeing as the manual cannot.
C&G 2377 -01 & -02