Restricting values in drop down

To restrict values in drop down -
We have to use get_att_v method and use ddlb_reason attribute in context node class. And this drop down is generally in the header so we have to add this attribute in the CN00 class.
A private variable with ddlb_reason is created which is used to pass on the drop down values.
http://scn.sap.com/message/10042735
Now lets say we have field1 when a specific value is selected in field1's drop down it should display restricted values in field2 drop down. For this write a logic in the get_v method of field1 to populate into ddlb_reason field (which should be displayed as a static attribute). This attribute can be accessed in the get_v method of field2 and be displayed.

To create and F4 help -
This can be done in any context node. And all this needs is rv_valuehelp_descriptor. This has a standard code. And is implemented in the GET_V_attr method.
You need to have the attribute technical name, attribute name, search help name, search id type. Search ID type can be from DDIC - HELP_ID_KIND_NAME
                           Reference structure field - HELP_ID_KIND_COMP
                           Reference data element  - HELP_ID_KIND_DTEL
http://wiki.sdn.sap.com/wiki/display/CRM/How+to+provide+F4+Help