The empty state for your Fork Parent if you don't have any branch roots defined is Latest Simulation. Naturally, in the Forks tab, you can choose any of your previous Fork Simulations and mark it as a branch root so you can use it as a Fork Parent in your Fork Simulations.
Marking your Fork Simulation as a branch root will add it to the list of Fork Parents you will be able to use going forward (and you can remove it as a branch root at any time).
Branch roots will allow you to define any number of fixed points when forking networks so you can run Simulations from that or any other Parent.
You can also name your Fork Parents for easier navigation:
Now, when you run a new Fork Simulation, you will see all of your Forks that were added as branch roots:
All of the Fork Simulations that were ran with a Parent will be clearly marked:
If you clear the branch root from a Fork the Parent column will be properly updated:
If you mark a Fork as a branch root again after removing it, the Parent column will again be properly updated:
You can also mark Forks as branch roots straight from the Transaction Overview tab: