Pinaki,
Suppose a customer in ECC has number 100010
Now when it is replicated to CRM, it will have a number 3000011223 for example.
When you look into the different Ids of this CRM number, you will see EIN, BSSN etc..
What system is telling you now is that it has to store the ECC customer number against ID type CRM002 so that it knows which ECC customer is mapped to which CRM Business Partner. So I think you should allow this ID type.
Niraj