According to our partners at Real Life Software (authors of ServiceLife) :-
ServiceLife 5.4 & 5.5 links to Sage Line 50 and Instant Accounts
With earlier versions of Sage Instant Accounts, you can only read information into ServiceLife.
It's a way of importing the customers and suppliers in one go at the start.
With Line 50 (and Instant Accounts V19 onwards) you can create customers, suppliers and, most important of all, invoices.