There may be times when you are out of a particular item. This is known as a "stock-out". Rather than lose the sale, you can set up a substitute for that item. Go to More, Substitute.
To create a new substitute, click the "Add" button on the bottom left.
Then, select the item you want to have as a substitute and click "Continue".
A dialog box appears, asking if you want this substitution to be reciprocated. That means each item is a substitute for the other.
A checkbox indicates if this a reciprocated substitution.
To navigate to a substituted item, click the left carat icon.
To delete a substitution, click the x icon. If the item is reciprocated, then you'll be asked if you want to delete the substitution for both of these reciprocated items.
The process is the exact same for Upsells.
The advantage of having a Substitute and Upsell data is during the sales process. On an Estimate Order or a Sales Order you have the option of selecting a substitute item or an upsell item.