If an SD has two different values for "summary" and "snippet" in its esriinfo/iteminfo.xml, the "snippet" value is displayed for both "snippet" and "summary" on the service's endpoint info/iteminfo HTML page.
Here is what is contained in the SD's iteminfo.xml relative to this problem:
<snippet>snippet text</snippet>
<summary>summary text</summary>
When the SD is published and its iteminfo page is displayed, you'll see:
snippet: snippet text
summary: snippet text
I'm fairly confident this is a long-standing bug in the ArcGIS Server's code when it's formulating an iteminfo HTML page response. I can see the same behaviour in our 10.4.1 server and am currently working with 11.1 Looks like a copy/paste error, probably from the code's inception.
Just letting Redlands know in this forum as well as the Support case I just submitted. Picked "Enterprise" because I couldn't find anywhere to pick Standalone ArcGIS Server.
