I agree with the avazula's answer that the difference between a party social event and a non-party social event is that a party is always meant to be a joyful occasion. However, I don't think this is significant enough to warrant a separate tag.
To give some examples, parties can include:
- house party
- frat party
- dinner party
Not necessarily parties, but still a social event:
- family reunion
- picnic with friends
- charity dinner
- poetry slam
(Side note: I deliberately left out things like weddings and funerals, because those events have a well-defined cultural significance and etiquette associated with them, unlike these examples.)
The distinguishing factor that makes a time spent with others a "social event" seems to be that it involves some amount of preparation, planning, organizing, inviting, etc. So then, the only real difference between a "social event" and a "party" is that parties are thrown with the main goal of having a fun, often celebratory time (whereas a more general "social event" might have a main goal of raising funds for charity or sharing family news). I don't think we need a separate tag for that.
Reason 1: Either way, the body of the question will still need to contain the details of what the occasion is and what the expectations are for guests at the event. Calling it a "party" doesn't really give us any more information than "social event".
Reason 2: From the tag help page,
Tags are a means of connecting experts with questions they will be able to answer by sorting questions into specific, well-defined categories.
Tags can also be used to help you identify questions that are interesting or relevant to you.
I think "social events" and "parties" are close enough in nature that someone who is knowledgeable about one will also be knowledgeable in the other, so again, there's no need to treat them as different types of questions.