Reset
What is the reset button for?
The hierarchy uses a number of interface records (5001 of them) to draw the hierarchy portal. These don't contain any data and they don't really have any "weight", but they are important. If you accidentally create or delete records in the hierarchy table you could get gaps in your hierarchy. So should such records get added or deleted you can use the reset button to restore the original 5001 records. You can find this (very small) reset button in the lower right corner of the home screen (click the house icon in the upper left of any screen).
Preventing resets
You'll want to prevent your users from creating or deleting records in the hierarchy table. You can do this through FileMaker's built in access privileges (just be sure users can still edit hierarchy records) or through custom menus be remapping the New and Delete menu commands to your own scripts.
