the statement is aggregating "Gross Sales Amount" into a module that contains (at least) "Product-customer" and "contract name" as dimensions.
The lookup part of the formula refers to a line item in a module that contains a value related to a dimension in the destination module (not actually a dimension in the destination), AND is a dimension in the source module (i.e. Import Rebate Actuals).
The sum statements always refer to dimensions in the destination (module). If the dimensions mentioned in the sum statement don't exist in the source (module), then they can often be defined in line items (in the source, or in a module that has dimensions aligned with the source)... for example "Import Rebate Actuals Map.Reporting GroupProductCustomer List" looks like a line item from a mapping module that is, at least, partially aligned with dimensions in the source... which allows Anaplan to (effectively) treat the two modules as if they were one for purposes of formulas. Mapping modules can be very handy provided they are used in a way that will scale well. I was at a client, for example, that created a variety of mappings that were contained in modules that weren't versioned, but were then embedded in formulas that applied different mappings to different versions... this is (and remains) very messy and difficult to work with... its on our cleanup roadmap.