Registry files and the Salesforce Instance drop-down work together to ensure scenarios executed by JobBuilder are processed in the correct instance with the correct settings.
Why use a registry file?
Registry files hold settings from the login tab (username, password and proxy settings) as well as settings from the DemandTools' Options menu (general settings, file paths and CSV file options).
Often times consultants or users who log in to multiple Salesforce Orgs find themselves with the tedious task of updating these settings every time they switch. This can be avoided by setting up a registry file for each instance being logged into that can be initiated with one click.
If we look at .reg files and how they impact a scheduled job, the benefit is the ability to jump from org to org within a batch with the confidence that each scenario executes under the best settings for every instance or job.
When using Registry files, we highly suggest a Salesforce instance is specified for each scenario to run in as opposed to using the "DemandTools as last Logged in" option.
For example, each line item of the batch should have a specified instance.
By using a .reg file and the Salesforce Instance feature there is no way a scenario will be accidentally executed in the wrong instance/org.
For more information on creating .reg files, click HERE.
For more information on the Salesforce Instance feature, click HERE.