Steps of enabling Item Substitution.
- Enable Multi Inventory Location feature in the account.
- Install the Supply Chain Management SuiteApp
- Enable Item substitution feature in the account.
- Give permissions to roles -> Custom Record–Item Substitution — Full
- Also check the checkbox Enable Item substitution in the employee record
- To access Item Substitution for roles – Item Substitute Transactions CS script -set the audiences to all roles you want
- In the item record, under the Item Substitution subtab, add the substitute items for an item (any number of items can be added)
- In the sales order form, go to customize form and enable – Item Substitution and original item.
Steps to apply Item Substitution
Set Substitute Items for an item in the item record itself.

Then create sales order with this item for which we have chosen a substitute item.
If the item is not available, then the item substitution field in the item line level will show an alert symbol.

On clicking the alert symbol,

Choose the item which is needed as substitute for this

The item changed to substitute item

The original item field is pop up with original item and part number pop up with substitute ite. Also the symbol means that the item is enough in quantity.
Then save.