Regarding Incompletion Procedure 
( Incompletion Procedure is exclusively for SAP-SD ) 

Places where we can maintain the Incompletion Procedure;

1. Sales Document Header.
2. Sales Document Item Category.
3. Schedule Line Category.
4. Delivery Document Header.
5. Delivery Document Item Category.
6. Sales Activity.
7. Partner Function

The steps are as follows;

1). Create Status Group 
( Status Group is nothing but the area you want to get effected if end user missed some entries ). 
The T.Code to create Status Grp is OVA0.

2). Define Incompletion Procedure by selecting fields as per your Client's specification.
Like for example in delivery Document we can say that Ship to party is must so in Sales Order we have to make the Incompletion Log that include Ship to party than only we can create Delivery Documents. 
T.code is OVA2. Here you can define the massage type if you miss some entries like warning msg., error msg etc....as per the importance of the fields.

3). Assign Incomp. 
Procedure to required field like for Sales Doc., the T.code is VUA2 etc.. 
e.g. Incom. pro. 11 is for Sales Order so you have to assign that 11 via VUA2  to Sales Order. 
 

How to identify the screen name in Incompletion Procedure?

First you have to identify in which particular tab that incompletion log should work for you in sale order.

For example, if you want a control at header level in sale order for the field Pricing date, you have to check which table and field the value will be stored. In this case, it would be VBKD table and PRSDT field. As I said above, this will come in tab "Sales".

So now in OVA2, after selecting "Standard Order" double click on "Fields". There maintain the above table and field and in the Screen tab, just press F4 so that you can see "Sales: Header - Sales". This would be the appropriate screen for the control you want. Like this, you have to assign accordingly.

See Also
Steps for Partner Determination Config in IMG

SAP SD Books :-
SAP Sales and Distribution, Interview Questions, Certification and Configuration Books

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