Identify Source document type attribute name in OM extension

Summary:

We have requirement to write an OM extension based on source document type. However, i could not find the attribute name that represents source document type from sales order Header.

I got the list from extension attributes from oracle doc however that does not have the source document type.

Can you please let me know the attribute name that can be used to represent source document type?

def SourceDocType = header.getAttribute("??");

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