Refer to the Exhibit. The Exhibit shows the components of a purchase order. The application requires that the total number of items (TotalItems) in the Trailer Record equals the sum of the quantity…

TotalItems is a single field within the Trailer Record and does not have the scope to iterate over or sum QtyOrdered values across multiple Detail Records.

QtyOrdered is a field within an individual Detail Record and only has visibility into that single line item's quantity, not the trailer summary.

A component rule placed on the Trailer Record has access to TotalItems within the trailer and can iterate over all subordinate Detail Records to sum the QtyOrdered values, giving it the full scope needed for this verification. The Trailer Record is the parent container that logically owns both the summary field and the relationship to the detail data. This is the appropriate level to enforce a rule that compares a summary field against an aggregated value from child records.

Placing the rule on the Detail Record would only validate individual occurrences and lacks access to the Trailer Record's TotalItems field for comparison.