Your split tickets work exactly the same way as any other Hatfield to London train tickets on the National Rail network do.
Instead of having one train ticket to take you from Hatfield to London the journey is broken down into two or more parts with a separate cheaper ticket for each sector where your train would ordinarily stop. You are still able to reserve your seat, enjoy additional discounts if you have a railcard and you do not have to change trains at the stations where you have split your ticket. For more information on Hatfield to London split train tickets follow the links in the menu bar below.