Exit Codes
adTempus does not know what, if anything, a particular exit code from your program means. It only reports the information it receives from the program, and responds according to the rules you have defined.
The meaning—if any—is determined by whoever wrote the program.
Whenever a program runs, it passes a numeric exit code (or return code) back to the operating system (or in this case adTempus) to indicate its status.
While all programs return an exit code, not all programs return a meaningful exit code.
By convention, programs return a value of 0 to indicate successful completion or some value greater than 0 to indicate a warning or error condition.
Generally, programs that were designed to be run in a batch fashion will return some sort of useful exit code. This value can be used by adTempus to determine the success or failure of the task (see the success criteria for the Program Execution task).
adTempus does not know what the exit codes from a particular program mean, if anything. Check the documentation for the application in question, or check with its maker.
Remember, it's entirely possible that the exit code is meaningless. adTempus defaults to treating anything other than 0 as a failure, because that's the convention. If you program appears to be working properly but keeps sending back an exit code of 42, you can change the success criteria for the step so that adTempus won't fail it.
If your program does not return a meaningful exit code, you can configure adTempus to ignore the exit code, or use the Success Rule Script to determine whether the program succeeded by reading an output file from the program or checking some other indicator.
How to Set the Exit Code
How you set the exit code depends on what development language you're setting it from.
C/C++ Programs
The exit code is the return value from the main() or WinMain function.
.NET
If you are using VB.NET or C#, you need a main function that returns a value; that's the exit code. See the .NET documentation for details.
Visual Basic
If you are using Visual Basic .NET, you need a main function that returns a value; that's the exit code. See the VB.NET documentation for details.
If you are using an earlier version of VB, there is no supported method
for returning an exit code. Microsoft used to recommend the approach described
below. We give it to you here for your convenience, but please be aware
that they no longer recommend or support this approach, due to the problems
described in their knowledge base article Q288216. 
Show me how.
In addition to the problems noted by Microsoft, we'll point out that calling the ExitProcess function while your program is being debugged in the Visual Basic development environment will cause the Visual Basic IDE itself to terminate, along with your program.
A better approach would be to a) use something other than VB, or use a newer version of VB or b) use a file to signal success. You can then use a script to check for the presence of this file and set the step's status accordingly.
Batch Files
The exit code from a batch file is the exit code of the last process the batch file executed. If you want to set the exit code of your batch file to something in particular, use the exit command:
exit /b 42
Sets the exit code to 42.
Scripts
If you're using a Script Execution task to run the script you can return an exit code as described here. If you're executing the script in some other fashion, you can use the WScript.Quit method to return an exit code.
Everything Else
Check the documentation for whatever development language you're using.