In a typical customer invoicing relationship: Customers to Invoices to LineItems to Products

I can put a portal on a customer layout to see what products they have bought (portal connects to Products table). But how would I get the quantity of each product they have bought to display in that portal?

You need another relationship between Products and LineItems, which will include only line items of the customer you are currently looking at. Assuming there is no CustomerID in LineItems, this relationship needs to be:

Products::ProductID = LineItems 2::ProductID

AND

Products::gInvoiceIDs = LineItems 2::InvoiceID

Products::gInvoicesIDs is a global Text field that you need to populate by a script triggered by loading a record in the Customers layout, setting it to =

List ( Invoices::InvoiceID )

You need another relationship between Products and LineItems, which will include only line items of the customer you are currently looking at. Assuming there is no CustomerID in LineItems, this relationship needs to be:

Products::ProductID = LineItems 2::ProductID

AND

Products::gInvoiceIDs = LineItems 2::InvoiceID

Products::gInvoicesIDs is a global Text field that you need to populate by a script triggered by loading a record in the Customers layout, setting it to =

List ( Invoices::InvoiceID )

& to grab the data, do you just use the new Table Occurrence just created with that relationship or this would be a multi-predicate join only?

Both.