Sum Index Field Technology (SIFT) is a built-in technology that exists in Microsoft Dynamics NAV and is used for totaling. It seems that Microsoft bought Navision for FlowFields, which are considered to be a brilliant feature.
In other ERP systems, totals, subtotals, and balances are calculated and stored somewhere. This calculation has to be redone over and over so that numbers are up-to-date.
In Microsoft Dynamics NAV, if you're a developer, you don't have to worry about calculating subtotals, as it can be done through SIFT. Creating a new Subtotals field is as easy as indicating in the field properties that it is FlowField and specifying the formula in the field. After this, you will not have to worry about keeping it up-to-date.
As a user, you know that the balances for your G/L accounts, customers, vendors,...