When you are setting up a business rule, you are able to add a "Sections/Subsections" link in the initial setup:
The Sections/subsections is a list of specific data areas in the Application Data, and it tells the rule 'where' in the Application Data to run the logic against. When you click on this dropdown, you get a list of all the sections/subsections of the Application Data, and you can click (Choose) one or more of these sections to tell the rule 'where' to run.
Note that you can add more than one Section/Subsection - this will mean that your rule will run against Application Data in more than one place.
eg. Let's say a lender does not allow an application to have more than 1 security property in it. To ensure that valid applications are submitted, the lender adds logic to check that only one Property Asset is noted as a "security". Given Property Assets can be added in two places (both the Financial Position and the New Securities section) - the lender sets the Section/Subsection to check that Property Assets in both sections:
