The protected DataLink property is used internally when the SQL dataset acts as the detail dataset in a master/detail relationship that is not based on nested datasets. The datalink object manages communication between another (master) dataset and this SQL dataset. For example, it is the datalink that tells this SQL dataset to refresh its data when the active record of the master dataset changes. 

Do not use DataLink to establish the relationship between an SQL dataset and another (master) dataset from which it obtains parameter values. Instead, use the DataSource property, or, if the dataset is TSQLTable, the MasterSource property. 

DataSource 

TSQLDataLink 

MasterSource