How do I sort idented data item in a report

I am new to C/AL and need help in modifying a report.

The report uses two tables - the first table, Contracts and a second indented table called Comments.

The DataItemLink between the two tables is No.=FIELD(No.) so I know that only Comments whose No. = Contract. No. is extracted.

I need to extract only Comments

WHERE(Table Line No.=CONST(0),Document Type = CONST(Pool))

My questions

Where do I insert this code ?

Do I have to use C/AL code like SETRANGE in the PreDataItem trigger in the Comments table ?

Thanks in advance - all suggestions are welcome!