References For Learning & Using Applescript
Learning & Using AppleScript & JavaScript for Automation (JXA)
I'm not sure there is a good definition of a splitter group anywhere...
GUI-Scripting with System Events is complicated and takes mucho patience to learn. 
You need Apple's Accessibility Inspector (part of Xcode), or preferably UI Browser ($55.00 U.S – there is a demo.). You need to know a bit about AppleScript, and you need some mentoring.
Either utility is able to highlight UI-Elements in an app, so it's easier to associate the name with the element. The syntax of all the items can get really gnarly though.
I bit my tongue about 10 years ago and bought UI Browser 1.0; it has me hours of time and kept me from pulling my hair out.
The way to get an item out of a list is:
set myItem to item 1 of myList
-Chris