Sequence name used for Particular Document Type

I have this query : how do we know that which sequence will be used for document type. In Orace Service Contract, we have defined the Sequence in Document Sequencing setup: Define->Categories->Assign. We also have the Autonumbering setup in Service Contracts setup. But we have requirement were in we need to create warranty contracts(calling API's)  from a custom Program. Now I want to know if there is any sequence defined for Warranty contracts and if I can use the same sequence and generate contract number from backend code using Sequence_name.nextval. Please help  me knowing how this document type (Warranty Contracts