From the help files:

Back Ordered

Type the quantity to back order. If the quantity shipped is less than the quantity ordered and the appropriate security is set up, this field displays the difference.

If the quantity shipped equals the quantity ordered, this field displays a zero. A back order is generated automatically for this invoice in the Sales Order Entry file for any back ordered items.

If the Inventory Management module is integrated with Sales Order, the Quantity on Back Order field in the Inventory file is adjusted by the quantity back ordered.

If this is a one-step invoice, a back order is created in the Open Order file using the next available order number.

This field is available only if the Back Orders check box is selected for the item in Item Maintenance, and the appropriate security is set up.

The following pertains to SAGE 100 ERP:
It is our experience that the back order status occurs in one of two ways:
1) Upon sales entry, if there is a known back order, a sales order entry clerk can denote the back order quantity in the b/o column of the sales order. Then the remainder of the order gets shipped and invoiced at the shipping data entry level (typically at the warehouse) .
or 2) Upon shipping data entry when the shipping department tries to fulfill the order quantity, finds a shortage, and then back orders the amount short shipped.
In both cases, after this invoice rolls up to post (perhaps at end of day), the original work order is re-opened the next day, and the status of the order becomes "b" for back order. The system remembers how much remains to be filled against the original order quantity.
I will add through our experience that we discontinued the use of the the back order column at order entry as we would always want to fill whatever we had inventory for. The case for a discrepancy could be a newly arrived shipment (transfer or receipt) that had not been updated into inventory but is physically in the building.
We also found that order entry clerks began manipulating the back order column by changing the amount on the next release of the work order when trying to fulfill the back order which caused invoicing issues. Since the system recalls what still needs to ship to close out the order, it is best not to manipulate the b/o quantity when shipping the back order goods when they have arrived.
I hope that helps

We are still trying to figure out a good way to track the backorders and invoicing for shower accessories even though they haven't shipped. We've been experimenting with the "Drop-Ship" feature and creating our company as the vendor that will be shipping the item(s). This works dandy; however, we are still not able to invoice for the items that would be "drop-shipping". The whole reason we are trying to invoice in-full is that we don't like to show the individual pricing for optional accessories that are included in our showers.

For instance:

1 Shower
2 Grab Bars
1 Seat

Total: $1,358.00

Do you have any suggestion on how we could accomplish this?

Look (within the help files) at Kits (item Product Type) and options for exploding line items. You might need to customize your forms though.

We don't have the accessories kitted because there are so many variations and configurations. We enter in the specific accessory requested by the customer and roll-up the pricing into a "Main Item" to be quoted as one unit.

Then put a checkbox UDF to indicate the lines you want to suppress pricing, set the accessories to $0 price, and modify your form to hide the price when the UDF is checked. It will be tracked just like any other line item, just with the customer never seeing the price.