Manufacturing Process and Item Element Question

Hi,

My name is Carlos and IT Functional Consulting in Argentina.

I have been looking inside the pps-schema-1.0.xsd and I have some questions.

1) Why the element Item, it isn´t the same as the UBL element item?. Have I extend the pps-schema-1.0.xsd  with the UBL Item, if I want the same structure?

2) What Entity have I choose of pps-schema-1.0.xsd  If I want to represent a serial manufacturing or a production of a good, in which I want to express a BOM (bill of materials) for one item, like a computer. For example. I want to manufacture a Computer and a Computer has a 1 Keyword, 1 DVD player, 1 Monitor. Where I want to express that I want to produce a computer consuming  each part?

Do you have same xml examples or other documentation that can help me?

Thanks you very much.

Carlos.