Union(联合) 问题

#include <iostream>

union Holder
{
char holdchar;
short int holdint;
long int holdlong;
float holdfloat;
};

// Function prototype.
void DisplayHolder(Holder, char*);

int main()
{
Holder hld;

// Assign to first member.
hld.holdchar = 'X';
DisplayHolder(hld, "char");


hld.holdint = 12345;
DisplayHolder(hld, "int");


hld.holdlong = 7654321;
DisplayHolder(hld, "long");

hld.holdfloat = 1.23;
DisplayHolder(hld, "float");

return 0;
}

void DisplayHolder(Holder hld, char* tag)
{
std::cout << "---Initialized " << tag << " ---"
<< std::endl;
std::cout << "holdchar " << hld.holdchar << std::endl;
std::cout << "holdint " << hld.holdint << std::endl;
std::cout << "holdlong " << hld.holdlong << std::endl;
std::cout << "holdfloat " << hld.holdfloat << std::endl;
}

疑问:

1) tag 是什么 ?

2)不明白 --initialized char-- 的 输出 怎样來的, thank you very much !!!