user
usergroup
accesslevel
date/time
device (desktop/mobile/tablet)
country
language
ip
current page

and much more...

Any of the above AND all combinations.

example:
if($user_id){
//user is logged in
//redirect to frontpage
$redirect_url = 'index.php';
}else{
//user is not logged in
//redirect to login page
$redirect_url = 'index.php?option=com_users&view=login';
$message = "please login first";
}

Uses the PHP syntax, so you can custom script everything possible in PHP.