It will use to run your reports (ABAP programs).....
Usually, this transaction will be used for for the End-users,those who needs to run the some customizing reports ....becuase they will not be provided with the Developer authorizations to run the reports from Se38..or Se80...
with this Tcode we can assign transaction code to SQL query
Create a transaction calling transaction START_REPORT with the following parameters/attributes filled :
D_SREPOVARI-REPORTTYPE= AQ "parameter indicating Abap Query
D_SREPOVARI-REPORT= 'ZGRP' "Query User group
D_SREPOVARI-EXTDREPORT= 'ZNAME' "Query name
If you use the way i described earlier your transaction will actually call the query. (START_REPORT was developed for this and can call ABAP Query, Report Portfolio Rpt; Form, Drilldown reporting, Report Writer Report and Transactions with variant)
If you want to create a report from query to create report-transaction then you have to use the generated program or copy it to a Z-program, but beware that changes to the query may not be reflected to the transaction.
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<b>Reward i fusefull</b>