what purpose cbsize member win32api structs

i frequently confront definitions win32api structures (but unparalleled it) have cbsize member following example.

typedef struct _test {
    int cbsize;
    // members follow
} test, *ptest;

and following this:

test t = { sizeof(test) };
...

or

test t;
t.cbsize = sizeof(test);
...

my initial speculation potentially used versioning. dll receives pointer struct check cbsize member approaching value dll compiled. check repremand make-up finished struct. nonetheless i here you.

what purpose cbsize member c++ structures win32api?