Read Bound to determine whether a value has been assigned to the parameter. Whenever a value is assigned to the TParam object, Bound is automatically set to true. Set Bound to false to undo the setting of a value. The Clear method replaces the value of the parameter with NULL, but does not set Bound to false. However, if the Clear method is used to bind the parameter to a NULL value, Bound must be separately set to true. 

Datasets that represent queries and stored procedures use the value of Bound to determine whether to assign a default value for the parameter. If Bound is false, datasets that represent queries attempt to assign a value from the dataset indicated by their DataSource property. Similarly, when Bound is false, datasets that represent stored procedures attempt to supply a value directly from the server. 

Clear 

IsNull 

TQuery 

TStoredProc