Outlook Appointment/Meeting/All Day Event

they look the same to me.

they all have start -End time, subject , invite attendees, and the body of the message.
you can check the All day event  box.

the similarities are confusing...I do not know if there is something behind that

Its more of an availability option so this way you don't have to go back and remake it and can update it on the fly etc.

Example:  You have an appointment on your calendar which you are using as a reminder for yourself to go talk with HR, Something comes up where you are going to be considering other people involved, so instead of removing your individual appointment you can turn it into a meeting and send invites.  

If you just double-click on an empty spot on your calendar you are opening a blank template, if you right-click and choose all day event but it turns out it wont be you can change it.

When you create new Appointment, let's say to Haul some boxes, then you invite someone else to help you, then it will look just like creation new meeting or all day event

Thank you