Check date and check order

Question

Dear Experts,
Fairly general question.
A check instruction can have av. check first and alloc check next and vice versa.. with no apparent difference in results of the confirmed quantity in either case. (unless we bring in rules and remaning requirement logic that I am not at the moment).
There is small and sensible requirement.
I want to confirm a sales requirement against remaining allocation at a different date vis-a-vis actual availability check date (mbdat) against ATP quantity. For simplicity assume there is no b/w and f/w consumption in allocation design.
SAP help says this is controllable such in standard. but I want to know what could be the possible side effects and any other possible interpretation of such a design...keeping in mind this is change on top of live system where pre-sales and sales documents consume allocations without a strong concept of scheduling (within the check framework). Could this have any bearing on the order of check steps i.e. if I want to check allocations at a date earlier than material availability date, should Allocation check be the first step. I need to set up a hell lot to verify all this. help.com has no such re-assuring statement that I could locate.Hence looking for first cut answers from your experiences.
Business context: I want to check remaining allocated quantity on "scheduled" goods issue date as opposed to material availability date which is scheduled backwards from customer requested date. The reason being the customer has been promised a certain shipping date based on a auction he won. This means that being able to meet shipping date is more important than being able to meet scheduled delivery date. This is true of not all sales scenarios but only some. Moreover we take allocations seriously because that is the very basis for production planning from the limited raw materials from nature
Regards,
Loknath

Answer 1

Loknath,
I don't really understand most of what you have asked.  You have stated it all as theory and not given any concrete examples of how you would like the system to react under various scenarios.  However, I will answer as many questions as I can.
I need to set up a hell lot to verify all this
???? provided you already have a working SCM product check configured, and provided you do not have to create a custom characteristics, you can set up and test a single sequence combination alloc/product check ATP in less than a working day.  Once set up, you can switch the allocation/product check sequence in your check instructions in seconds.  The total amount of work will be the same, regardless of which sequence you end up using.  This is work you will have to do anyway to implement allocation.  If you check your planning area directly during development, you can vastly reduce the amount of time you spend on testing your theories.  In a day you will have created 90% of the final solution, and you will have tested multiple theories and scenarios.  The only wasted effort would be if you decided to abandon allocation altogether.
if I want to check allocations at a date earlier than material availability date
??? of the 3 dates available, MAD is the earliest.  What date do you intend to check that is earlier than MAD?
I want to check remaining allocated quantity on "scheduled" goods issue date as opposed to material availability date which is scheduled backwards from customer requested date.
???  Both dates (GID & MAD) are scheduled backward from delivery date.
There is nothing inherently right or wrong in using any of the 3 dates (MAD/GID/DelivDat).  When you are setting up your allocation quantities, you need to set them up in terms of the date within the sales order that you will be checking.  All the dates in your planning book will represent one of the three dates, as configured in your allocation group.
The most common reason to implement allocation is that order entry is expected to be far in excess of capacity. Under these conditions, when I set up allocation, I usually set up Allocation first, then product check.  I do this mostly to limit system load during backorder processing, since in most allocation situations, more orders will fail allocation check than will fail product check.  The product check on these orders never gets executed.
Best Regards,
DB49