There are several different ways that products can be organized in a catalog. The best one depends on the type and number of products and the catalog's overall branding identity.
For example, clothing items can be organized by type (tops, bottoms, sweaters, dresses, etc.), season (fall items, summer items, etc.), collections, or even fabrics/materials (leather items, knitwear, etc.).
Other products might lend themselves more to alphabetical listings, for example, if the products are ordered via an SKU, product code, or part number. This is also practical if there is a large number of items that you can list in chart format without needing pictures of each item. The latter tends to be used in more "industrial" catalogs where the product is being sold to meet a specific need, like parts for repairs.
Whatever way you organize the products, it should be easy for the customer to find what they are looking for. This can be done with sections. Each section can have a different coloured bar or background or some other differentiating feature.
Catalogs with sections will require a Table of Contents and clear, easy to read page numbers. They should be in a plain, easy to read font, and large enough that anyone can easily make out the numbers. The best page numbers are either a very dark colour on a very light background or vice versa (think black on white or white on black) so that the number does not blend in.
For products organized alphabetically, a Tabel of Contents can still be beneficial. It can tell the reader which page each letter sections starts on. However, an Index can also be helpful, since it allows for an alternate sorting of the products (for example, it could group parts based on their model).
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