Hi Piyush,
To add to Uday response, the common extension collection template has the following columns that needs to be populated with the column definitions created in your application:
| # Field | Type | Required | Notes |
| # CLASS | CUSTOM | true | The name of the class having the target extension collection |
| # OBJECT | CUSTOM | true | References the EXTERNAL_ID of the object having the target extension collection |
| # COLLECTION | CUSTOM | true | The name of the target extension collection |
| # <EXTENSION_COLLN_FIELD_NAME> | CUSTOM | true | Placeholder for the name of an extension field in the target extension collection |
Once the values are populated you should be able to import mass values into the collection extension object.
Regards,
Parankush