You can create customer item alternates customer by customer (changing the items) or item by item (changing the customer).
Steps
In the search box, search for Customer Item Alternate List and select the link to open the page.
Select the customer you want to set the alternate items for.
Select the item you want to define alternates for.
Determine whether the original item can be sold to this customer. If not, select the Block Sales field.
On the line page below, select and enter one or more substitute items.
If you want to set up the same values for different customers, copy the rows with alternate items, select the new customer, and paste the selected rows.
If you want to set up more alternate items for the same customer, change the item number in the Sales Item No. field and repeat steps 4 and 5.