I would like to have the column F in the attached file for a display name of this list (concatenate "Product" + "PIC"). But the problem is since "Product" is a numbered list, I cannot have "Product" in TEXT format while I need to convert "Product" into TEXT to concatenate "Product" and "PIC"
As @VIGNESH.M stated, you can reference the Display Name of the Product numbered list. So if the numbered list for Product is Product Flat and if the display name of Product Flat is text, then the formula would be PIC &"_"&'Product Flat'.Display Name[Lookup:Product] as long as Product is list formatted. If it is not a list formatted, you will have to have the Product code accessible (in this list or preferably a module). This is another reason why having codes are so important, especially when using numbered lists.