The union declared without tag name is called as-

Answer 1

Unions without tagname are called anonymous unions.

Members of an anonymous union can be accessed as if they were declared directly in the containing structure or union. For example, given the following structure:

struct s {
int a;
union {
int b;
float c;
}; /* no declarator */
} st;

you can make the following statements:

st.a = 5; st.b = 36;

Answer 2

Untagged Unions which does not keep track of which union member is currently in use