How common is the character 唷 (pinyin: yō) in Mandarin? I have been searching for an official version of the Pinyin chart and I came across multiple variations of it. One of the charts I saw had the combination "yo". A quick search showed that one character it is used with is 唷 (yō).

The definition of 唷 is:


  (interjection expressing surprise) Oh!
  My!


My question is, why is "yo" omitted from some Pinyin charts? Is it a rare combination or is there any other reason? (Judging from the above definition, it seems like the hanzi is used frequently enough?)

Also, in this chart, yo is in the slot for "y+o" but in this article, it mentioned as a combination of i+o. 

That is but one inconsistency. There are many more differences from one chart to the other (For example, some charts don't have "yun" but there are clearly hanzi that use it). Is there an official set of combinations? (I found some compilations of the combinations but I don't know if some initial-final combinations are in use now.)

P.S. I read an article about "er" being part of a group of four special vowels and about how erroneous it was to place it in the chart with other pinyin finals. Does anyone know more about this? I can't find the link for the article so I don't have enough information to create a new question.
Major dictionaries published in mainland China usually have the official Pinyin chart as appendix.

You can download the official pdf of the Pinyin system from the government site: http://www.moe.gov.cn/ewebeditor/uploadfile/2015/03/02/20150302165814246.pdf 

You can also find it on wikisource: https://zh.wikisource.org/zh-hans/%E6%B1%89%E8%AF%AD%E6%8B%BC%E9%9F%B3%E6%96%B9%E6%A1%88

You also need to have a basic understanding of Chinese phonology and the Pinyin system. For instance, when there is no initial, finals beginning with 'i' should be written as 'y', and finals beginning with 'ü' should be written as 'yu'. Hope this clears your questions about yo is in the slot for "y+o" but in this article, it mentioned as a combination of i+o and some charts don't have "yun" but there are clearly hanzi that use it. 

In my personal experience, the character 唷 is not very commonly used. 哟 is perhaps more common. 

On the whole, interjections are often exceptions from phonological rules. (Think of the English word 'hmm'.) In Chinese, the pronunciation of interjections are often fluid. If a sound combination is only used in interjections and no other character uses this sound combination, it could be considered not part of the normal phonology and thus is not included in the Pinyin chart.