About 别了 and 再会
I have been learning Chinese a long time and have come across lots of ways of saying goodbye, but never these. The first question that springs to mind is what is the difference between 再见 and 再会。Is one consider more formal or polite for example? And what about 别了? It sounds quite casual to me, I think? Are both fairly common?

Can you tell me a little about when and why you would use 别了 and 再会 instead of other ways to say goodbye.

Question

About 别了 and 再会
I have been learning Chinese a long time and have come across lots of ways of saying goodbye, but never these.  The first question that springs to mind is what is the difference between 再见 and 再会。Is one consider more formal or polite for example? And what about  别了? It sounds quite casual to me, I think? Are both fairly common?

Can you tell me a little about when and why you would use 别了 and 再会 instead of other ways to say goodbye.

Answer

其实再见、再会都差不多。
“再见”在平常对话比较常用，“再会”则更正式一些。

除了直接說再見、掰掰，如果有指特別时间，一般会说
“我们明日见。” 或 “我们明日再见。”
但不太会说“我们明日再会。”

而“别了”有这辈子再也不见的意思。
（“永別了”也行）

Answer

the three words  have  same meanings ,
     the  most  casual is 再见，
     the second  is 再会，this words suit for fomal letters or email 
     and 别了 sounds  old fasiond ,it's meaning  is a little different from the frontal two words, it didn't   has the meaning that  you  want  to meet  your freinds again,so this words is more sad 
    For example:
      when i say goodbye to my  aquiantance   or  separate with my freinds,i would  use 再见
      When  i  commiunicate  with  my  business  partners  or  some  respectable  people,i  will  ues  再会
      when  i  know that   i  will never meet the  freinds again,in  this circumstances  i will  ues  the  word  别了to express  my sad mood

Answer

I would say 再会 is a tiny bit more formal than 再见, but 再见 is far more common in both spoken Chinese and written Chinese. I've probably used 再会 once in my entire life? As for 别了, it's truly never be used under any circumstance besides poetry. If we are saying goodbye, we'll just use byebye with friends and 再见 for strangers on the phone, sometimes use 好，先这样(also on the phone). Hope that helps;)