db.GetDocumentByUNID的问题

我在查看一个Notes数据库的Agent源码时，看到

Dim WorkSpace As New NotesUIWorkspace

Dim Session As New NotesSession
Dim Curdb As NotesDatabase
Dim uidoc As NotesUIDocument
Dim Collection As NotesDocumentCollection
Dim var,varn,varp,vars,vara ,varno,varino,str_business,str_account_no As Variant
Set Curdb = Session.CurrentDatabase
Set Collection = Curdb.UnprocessedDocuments

'doc--the selected Document;docp--the parent Document
'docn--the compose Document
Dim doc,docn As NotesDocument
Dim docp As NotesDocument

Set doc=collection.GetFirstDocument()

If Not doc.IsResponse Then
Msgbox "请选择入库单文档"
Exit Sub
Else
Set docp = Curdb.GetDocumentByUNID( doc.ParentDocumentUNID )
str_business=docp.business(0) '供货单位
str_account_no=docp.account_no(0) '帐号
End If
其中：Set docp = Curdb.GetDocumentByUNID( doc.ParentDocumentUNID )这条语句，doc本身不是答复文档，它没有父文档，那么这条语句起什么作用呢？但用Lotusscript debugger查看确实又取得了docp。
或者那位大侠能告诉我
Set doc = db.GetDocumentByUNID( doc.ParentDocumentUNID )
这条语句是不是根据父文档的UNID来取得当前文档的UNID，这条语句的实际作用是什么？
谢了！