Need to pass selected value of a table-based value set into another table-based value set

Summary:

We have requirement to pass the value of table based value set : XX_BV_CONTRACT_H (contract header) into another table based value set XX_BV_CONTRACT_L (Contract lines)

Please let us know the syntax to pass the value of one table based value set into another table based value set.

Refer to screenshot attached.

Also do we have a limit on the number of records to be fetched and displayed in a DFF value set?? We have created the value set XX_BV_PROJECT to fetch all the project numbers available for a particular business unit and associated to a DFF. There are more than 18000 records available but only a few are being displayed in the drop down for the DFF.