Selection Screen Behavior Automatic Filtering the values.

Hi,

I need a small information.Please have a on below attached images.

In selection screen i took two parameters like below.

                S_SDDOC FOR EKKN-VBELN, "Adding On 05.11.2015.

                S_SITEM FOR EKKN-VBELP ."Adding On 05.11.2015.

If i click on  S_SITEM  All the values are coming  from the data base.

Before selecting the line item if i give the sale order number then if  I select the Line no F4_ automatically filtering. May i know why.

Is it default behaviors. Automatic filtering means.  The sales document line no's are coming what i chose the  sales order document number.

There is no Filtering the Fields F4_help request.