A plan can be a place, occasions, events, subjects, landmarks, a business, hobby, interest, facination, subject matter etc. All posts in it will be about it
Plans have Geotag if the creator wanted it to, so users cab identify plans created near their location, incase someone is looking for whats happening around them. Business can use this to advertise to anyone around them.
Plans can be Private if the creator prefers. this means they are the only one allowed to make posts into it. Plans can also last forever or for 48 hours. ideally if a user create a plan to promote an even this plan should last 48 hours. allowing enough time for the happening community to view post in this plan before its remove from happening.
There is no one way to use happening.
You can use it to promote events, parties, weddings, conferences, kids events. To see "what the plan is" or "whats happening"
You can use it to showcase national landmarks. To showcase your business services or products if your an entrepreneur
If you love taking pictures and categorizing them by subject matter, you can also use it
Or just create a "forever private" plan for you to express your own individuality