semantics of dynamic varobj

Nowadays, if a varobj's contents are provided by pretty-printer, it is 
called dynamic varobj.  However, it is unclear to me what is a dynamic 
varobj?  What is the "dynamic" part of dynamic varobj? the children?

IIUC, the children of dynamic varobj vary.  That is why returned 
attributed 'numchild' of -var-create is "not necessarily reliable", right?

I ask these questions because I am adding a new kind of dynamic varobj, 
whose contents are provided by only available data.  For example,

struct foo
{
   int a, b, c;
};
struct foo foo;

foo.a and foo.c is collected in traceframe #1, while foo.b is collected 
in traceframe #2.  We create a varobj foo for variable foo, if 
traceframe is #1, foo has two children (foo.a and foo.c), if traceframe 
is #2, foo has one child (foo.b).  IMO, varobj foo behaves like a 
dynamic varobj.

-- 
Yao (齐尧)