Business question for clarification:
 
Component A (proc type E in mat master) has three components B,C,D assigned to it in BOM. B,C,D have reference points. With CN33, I assign these components, to an activity. So the three components could be seen below this activity. Procurement type in all three comp is F i.e. external procurement. 
 
I saw this example in IDES project T-20000. I was wondering as why should it be like that. Because, comp B,C,D will be procured externally and get consumed directly in activity. There was no need for comp A with proc type E. 
 
MRP type in comp A is PD with strategy group 70. When MRP was run at project level, nothing happened to this material, i.e. mat A. Comp B,C,D could be procured the purchase req. because of external procurement F in mat master. 
 
Business Logic explanation:
 
Since comp A has not been assigned to any activity, no planned order is created. if no planned order then no production order also. This means comp B,C,D will simply be procured and consumed in activity while comp A will be simply useless. 
 
So the clear question is: why are components assigned to activities at all. In my view, only header should be assigned if the need to be produced in-house. The components will be procured/or produced through production order. 
 
If something needs to be procured externally then it will surely need not have a BOM and hence can be directly linked to activities for later consumption.
 
Answer:
 
You have a production BOM consisting of three components. So material A will be produced by your project and this is why the procurement type is 'E'. The components for it will be purchased externally - hence the procurement type 'F'.
 
SAP has given two options to handle manufacturing materials. 
 
1) BOM - Planned Order -- Production Order -- Final Item. 
 
2) Assembly in Network. 
 
The IDES T-20000 case is an example of 2nd point. 
 
When you use case 2. It means that you are assembling in project network instead of in Production Order. 
 
In CN33 transaction, if you click on the 'transfer Button" (3rd from left at the top), enter project def., header material (i.e. A) and reference point, then header is also transferred to project structure. Header will be transferred with negative quantity and with black flush indicator flagged. All the child components (B,C,D) are transferred with +ve qnty. 

After you procure B,C,D issue them to the Network reservation. Now, child components are consumed. When you confirm the activity to which A is assigned , it will be posted to the WBS stock (if it is individual material) else to plant stock (if it is collective material). 
 
The conversion of B,C,D to A , is mentioned in the internal activity, you can mention the work center, duration in which B,C,D are converted to A. 
 
The above process is adopted when entire procurement, assembly and inventory is managed thru P.S. 
 
Else you can assign header material (A) to network activity and run MRP, which will create Planned Order for A and P.R. for B, C, D. 
 
So, the above are the two options given by SAP. 

Get help for your SAP PS problems
SAP PS Forum - Do you have a SAP PS Question?

SAP Project System Books
SAP PS Books - Certification, Interview Questions and Configuration

SAP Project System Tips
SAP PS Tips and Project System Discussion Forum

Best regards,
SAP Basis, ABAP Programming and Other IMG Stuff
http://www.erpgreat.com