Online store can have products related to each other. Let's say a
customer wants to buy a cell phone, most likely one would like to
purchase a charging device, memory card and a car kit along with it. So
it would be good to show customer these products as well. In Zen Cart
related products are represented as Cross Sell or X-Sell products. It
does increase sales, since customers may purchase even more than they
ever planned. So, Store Manager for Zen Cart simplifies the task of
creating related products. So you can link products to each other to
suggest items that customers might also wish to purchase.
By default product page may have the following look:

Main product shown above is Unreal Tournament. Note the section below stating "We Also Recommend:" - it shows related products.
Related products provide more convenient way for store visitors to
find what they would like to see, or maybe wouldn't, but what will be
useful for them. It does increase sales, since customers may purchase
even more than they planned before.
Requirements (Cross-Sell contribution)
X-Sell products will be available if you have X-Sell Products Zen
Cart contribution Installed. Otherwise your X-Sell Products Page icon
will be "greyed out", meaning the page won't be available.
General usage
You can manage Cross-Sell (X-Sell) products in two Store Manager
areas. First one is Product edit window, in "Products X-Sell" section.
More sophisticated way to manage X-Sell products is to do it in a separate section.

Here you can see all products at once and also related products assigned to them.
When you click on Add X-Sell icon, standard Search products form appears.

If you type a few letters of Product's name or model in "Text to
Search" field, the Manager will show you a list of products that match
your request. You may wish to specify a smaller number of rows to
display results, which speeds up the search. Unless the product you're
looking for is listed in the search results, please specify greater
number of rows and Click Force Refresh button. If you double-click on
any product in the search results, it'll be added to current product's
X-Sell list.
You can also set the priority for each product's X-Sell. If you look
at the example, you'll see that for "Unreal Tournament" product "The
Wheel of Time" X-Sell product is more important than "Disciples: Sacred
Lands", and will be shown first. You can edit Sort Order whenever you
wish to change their position.
In addition to add and remove this section also have some advanced features:
- Reverse Cross Sell (X-Sell)
This feature simplifies the task of creating related products. Say
you have products B, C and D which are related to product A. Most
probably (but not always) product A will be related to B, C, D products.
You can just add related products for A, and then automatically set all
reverse relations. Just select product A and select one of the options
from the menu:

- Smart Cross Sell (X-Sell)
This feature allows you to create related products for one or more
selected products automatically, analyzing customers orders. It goes
through all orders, searching products that were purchased together (in
the same order). These products will be considered as related to each
other depending on several options which you may configure:
- Max X-Sell items count - assign no more than
specified number of related products to given product. In other words
this tells Store Manager not to add new related products if some product
already contains specified number of related products.
- Minimum orders count - consider items as related only
if they appear in specified number of orders. Say you have specified 2
there - Store Manager will set products as related only if 2 or more
orders contain these products.
- Start orders analysis from - orders made before this date will not be considered as an X-Sell products source.