The following screenshot demonstrates a model which calculates a running sum:
This model makes use of parent referencing (the Data Structure element with blue border in the inner, repetitive process referencing the same Data Structure in the outer process). To create an parent reference, just drag the element to its intended location, right-click it and select type->Parent (note that parent reference is similar to ancestor referencing - an ancestor reference is a parent reference of a display element).
Note that the inner, repetitive process has a Control trigger which ensures that it is executed only after the Data Structure has been initialized.
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