user exit for Restricting Text id in sales order

Hello,

We already have a text id ZC50 in header sales text. we want that if there is header delivery block in sales order, then ZC50 text id should appear and if no header block in sales order,, then ZC50 should not be determined.

I read somewhere that RV70TNNN to be used but then this is data routine to be attached to access sequence in config. Access seq for a text id is used to copy text from previous documents to sales order but in our case ZC50 is to be manually input, so there is no access sequence for ZC50 and so I think we cannot use a data routine.

How can we achieve results where ZC50 text id is not determined when no delivery block in sales order ? which user exit ?

regards

sonu.