To add the “Reallocate Item” menu to the Sales Order form as a subtab in NetSuite, you can follow these steps:

1. Go to Customization > Forms > Subtabs > New.
2. In the Script ID field, enter a unique alphanumeric ID for the custom subtab.
3. From the Type list, select Sales Order.
4. If applicable, from the Parent list, select the parent subtab.
5. In the Name field, enter “Reallocate Item”.
6. In the Description field, enter a brief description of the subtab.
7. In the URL field, enter the following URL:

/app/common/item/reallocate_item.nl?id={id}

where {id} is the ID of the sales order.

8. Click the Save button.

Once you have created the custom subtab, you need to add it to the Sales Order form. To do this, follow these steps:

1. Go to Customization > Forms > Subtabs > Edit for the Sales Order form.
2. Under the Subtabs section, click the Add Subtab button.
3. From the Subtab list, select the custom subtab that you created in step 1.
4. Click the Add button.
5. Click the Save button.

Now, when you open a sales order, you will see the “Reallocate Item” subtab. You can use this subtab to reallocate items from one sales order to another.

Please note that you may need to contact your NetSuite administrator for assistance with customizing forms.