A 'Split Ticket' typically refers to purchasing multiple separate tickets for a journey that is cheaper overall than buying one ticket for the entire trip. For example if travelling from Edinburgh to Kings Cross, rather than purchasing a single ticket for your journey you may purchase split tickets, e.g:
Ticket 1: From Edinburgh to Peterborough
Ticket 2: From Peterborough to Kings Cross
Train Swap can be used with Split Ticket purchases if swaps are available on that route, and both fares are valid with Train Swap (Advance). As Split Tickets are separate tickets purchases, each ticket is treated independently. When it comes to Swapping you would need to swap each ticket booked in order to completely Swap your trip.