A Multi-Yield Work order is a single work order that creates multiple end items.  One example would be a work order to cut a plate of material (based perhaps on optimization nesting software that lays out a variety of different parts on the plate to optimize the yield) that when it is all done, several different cut parts of different quantities are produced.  
 
The first step to using the Multi-Yield work order is to create a type N "Placeholder" part number which is used to create the work order.  WHen you identify an item in IN-B as type N it asks if it is to be a Multi-Yield Placeholder.  Say yes.  
 
At this point in time, there is no BOM and the potential end products have not been identified.  In the example above, the "Placeholder" part may have a routing for the cutting process, deburring, and maybe even the initial engineering step of deciding what parts to make.  You can have as many different placeholder parts as you have different routing processes that would potentially apply.
 
Create a work order in WO-A for 1 of part "Placeholder".  Use WO-G to build the BOM by issuing the actual material used and collect labor costs against the routing.  Again, using the example above, the plate of material would be issued to the work order, engineering labor would perhaps be charged to the operation to determine the layout and then cutting and deburring labor would be charged.  
 
Once it is complete, go to WO-I.  When WO-I detects that the part in the work order being processed is a "Placeholder" part, it begins a different than normal process.  It asks whether the costs should be proportioned by weight, by Foot Factor or by Number of pieces.  Then it prompts for the specific items and quantities made.  Finally there is an option to redistribute the labor costs because you may not want the labor cost to use the same proportional logic as the material so you can manually redistribute it.  Then click Process and the items will all be put into stock at their proportional share of the cost.  If you are on Standard Costing or use Standard Cost in Enter FP, then the parts will go into stock at standard but the difference between the standard and their proportional share of actual will post to the WIP Variance.