Pluma's syntax highlighting

[Richard Owlett <rowlett@cloud85.net>]

I'm attempting to understand a shell script.
Pluma is apparently highlighting it based on the file extension (which 
is 'sh').

I thought I understood what its highlighting meant.
I didn't ;{
I attempted to do a web search and got primarily hits on people wishing 
to add highlighting which Pluma does not provide by default.

I am NOT interested in adding any new features.
What are the default rules for a file with extension 'sh'?
TIA