A valid script execution follows these rules:

1. It runs from left to right.
2. It continues until one of two things happens, either the script reaches its end naturally or an operation within the script signals a failure.
3. If the script completes without any early failure, the success or failure of the script depends solely on the top item of the stack.

In essence, a script is considered successful if it runs to completion and leaves a single "true" or any non-zero value on top of the stack.

Any other outcome - early termination, an empty stack, or a "false" value on top - results in the script being considered invalid or failed.