Summary indexing produces a lot of psrsvd_* fields. What do they stand for? I presume they're acronyms or abbreviations. Here are some examples when averaging the number of bytes returned per client IP, as logged by apache (ie 'sistats avg(bytes) by clientip'):
Here are some more for a count (in my case not literally, but basically 'sichart count by clientip')
You should accept your own answer. So that this question displays as solved. This information was very helpful.
Now on Splunk Docs:
https://docs.splunk.com/Documentation/Splunk/7.0.3/Knowledge/Usesummaryindexing