request for structure documentation

[NZG]

get_prev_register appears to depend on the value of frame_unwind->this_id,
but I can't find any documentation for what this element represents.
Could somebody point me to some documentation or elaborate a bit?

struct frame_unwind
{
  /* The frame's type.  Should this instead be a collection of
     predicates that test the frame for various attributes?  */
  enum frame_type type;
  /* Should an attribute indicating the frame's address-in-block go
     here?  */
  frame_this_id_ftype *this_id;
  frame_prev_register_ftype *prev_register;
};


thx,
NZG