Hi Ming,
If_wd_context_element is no where defined in component but its an interface used at runtime to read the context element reference of context node and is also used to get node's attributes.
Comparison:
Context node: is like table
Context element: is like a row/record in it
Attribute : is like a field
A context node can have as many as context elements inside it.
DD: From table ---> you get many rows/records ---> from each work area you can access fields
WD : From context node ----> you get the context elements ---> from each context element you can access attributes
Please refer to my answer in the below link
Difference Between Node and Attribute
Hope this helps you in understanding concept.
Regards,
Rama