I don't see "Helpful" in the list of things I can do when I press the flag link.
In my reputation I saw "6 helpful flags." How do I earn these flags? May I make them on other peoples' posts or comments?
The flag I am talking about looks like this:
That means that 6 of your previous flags were deemed helpful to moderators. You don't flag things as helpful yourself, you flag things that need moderator's attention. The helpful/unhelpful designation is our way of giving you feedback on the quality of your flags.
You can't. The "helpful flags" counter in your profile refers to the number of flags you issued that were deemed helpful, either by a moderator or by an event that occurred, like post deletion. If you click on it, you'll get an overview of the flags you've cast, and their outcome.
Flagging things is for bringing them to the attention of a moderator. Posts or comments certainly don't require mod review because they're helpful.
If you find something helpful, you upvote it.
Helpful flag simple meaning would be, Flag was helpful to keep the site clean.
After earning 15 or more Reputation, You get a link of "flag", which is for Moderator attention to check for the correct actions.
In the beginning you get "10 inform moderator flags".
After getting the flag intimation from a user, Moderator do research work for a particular flag. When Moderator founds correct information regarding a flag Moderator replies it with result "helpful"
On the right side of that page you can see all flag's report just like below image,
The Image says that there were total 23 attention flags out of which 18 were helpful , 4 were declined and 1 is waiting for review. In case of "decline" you get a message line from Moderator saying "declined - Sorry, had to decline ......"
Flagging is for flagging negatives - the tooltip even says "flag this post for serious problems or moderator attention".
It was never intended for positives - that's what upvotes are for.
Update, now that you explained in comments where you have seen "helpful" and "flags" together:
Moderators and high reputation users will see flags and if they agree with them will mark them as helpful (or not) - as in, they helped with moderating the site. It means "the flags from this user have been helpful".
If you found questions something like Spam, Duplicate, off topic, not constructive, not a real question, etc you can flag it.
If you found answers like Spam, not an answer, etc you can flag it.
If you found comments like rude or offensive, not constructive / off-topic, obsolete, too chatty, etc you can flag it
Moderators and reviewers will look at the issue and decide whether the flag was helpful or not and mark it 'helpful' or 'declined' or 'disputed'.
Also you can earn badges like