PATH models test scripts as documents, so the process of producing a test with thousands of users is akin to a mail-merge.
The DEF file (of which this is a sample), identifies line numbers within the script, what on these lines will be substituted, the data files and columns from which the substitution will be made, and whether a particular value should come from the current row in its data file, or whether a new row should be fetched.
During the initial stages of script development, we will capture the same script twice, and look at the differences between the captures, to identify the values that change from execution to execution.
We will conduct experiments to determine what needs to change, and what can safely be allowed to remain unchanged.
When changing values originated with the user rather than coming from the server, we will consult the domain index, to identify from whence valid values may be sourced.
Once the changing values have been identified, this file can be created automatically from the list of values and data files.
Manual tuning will be necessary if substitution has to change values like '1' or '2', which occur very frequently, and to sort out the F/N indicators.
Note that this sample DOES NOT correspond with the sample script used elsewhere on this site. The sample script still has the initial responses embedded in it. We strip these out of the the script that we deploy, to reduce the size of it, before testing with the script.