Product Mix

Product line, product mix, and product line extensions are terms frequently used to refers to different aspects of the range of products offered for sale by a company. I am not aware of a term like "product filling" associated with this. Also, it appears the term product extension in the question refers to the product line extension. I have explained below each of the three commonly used terms.

Product Mix:

Product mix, also called product assortment is the term with the most wide coverage. It includes all the products that are sold by a company. Some company may have just a few or even only one product in its product mix. For example a cement plant may sell just one type of cement. Other companies, such as a large chain of retail stores may contain tens or even hundreds of thousands of products in its product mix.

Product line:

All the products sold by a multi-product company at times can be classified in broad categories which are called product lines. The most common example is the different product lines that are usually sold by large retail companies like Wal-Mart. Typical descriptions of product lines used in retail industry include apparel, green grocery, dry grocery, household appliances, personal care items, and so on. A company may have just one product line in its product mix, Alternatively it can also have multiple product lines. For example, we have  retailers that specialize in only one line such watches, and white goods. WE also have store that sell from the same retail outlet both these lines of product, plus many more lines.

Product line extension:

Product line is a relative terms. It refers to addition of items to a product line beyond the previously sold items in the the line. For example, a company selling cosmetic products selling just one variety of cold cream, may decide to extend the cosmetic product line by adding two new varieties of cold cream that are meant to be used by people with extra dry and extra oily skin.