The mainlib would be pre-assigned and would be on the Server List. My question was not in reference to the output file (testing dataset), it was in reference to the source file - mainlib.monthly_data.
Also, I don't know the terminology, but there are 2 different ways a dataset appears in a project.
1. In the process flow - In my example, the output file would be in the project. If I resubmitted the same code, specifying a differently named output file, the 1st output file would no longer be in the project.
2. If I choose a file from the Server list and 'Add to Project', the file is permanently in the project, listed on left side - not in a process flow. The file is only removed from the project if I remove it manually.
In my original question, I was asking about Adding a file to a project in the 2nd manner - permanently.