CRM - ECC flow

Hi Experts,

We have a requirement to extract data from CRM to BI.

Please clarify few doubts related to CRM.

1. What is the common data between CRM and ECC.

2. What is the document flow from CRM to ECC i.e. which will be created first and why i.e. Prospects, Contract, Lead, Oppurtunity,Quotation etc

I am little bit confused why CRM is integrated with ECC.

Needs your inputs.

Thanks in advance.

Regards,

Bhadri M.