Question about namespace in kernel

[Leavesw]

in struct nsproxy several namespaces are defined:
utc_namespace
ipc_namespace
mnt_namespace
pid_namespace
user_namespace
net_namespace

Q1: When will a new namespace be needed? (examples preferred)
Q2: in Clone function, one flag is needed to tell the system whether a namespace needs to be created. but there are 5 flag bits but 6 namespaces. (mnt_namespaces doesn't have a flag) Why?

Thanks!

[Rubberman]

I'm looking in my kernel sources and don't find these symbols. Which kernel are you working with? In my RHEL kernel, namespace is define as follows:

struct namespace {
atomic_t count;
struct vfsmount * root;
struct list_head list;
wait_queue_head_t poll;
int event;
};

and none of the functions have any flag bits. Are you sure this is in the kernel, and not user-space? FWIW, my system runs a 2.6.18 kernel.