Search terms are not case sensitive, though field names are. A search like
sourcetype=WinEventLog error
That searches for a field named exactly sourcetype (it wouldn't match SourceType or SOURCETYPE, but will only match it if it's all lower case) for where the field sourcetype has a value of wineventlog. The wineventlog is not case sensitive, it's a search term, so wineventlog matches WinEventLog, wineventlog, or any other combination of upper and lowercase.
Ditto error. "Error" wasn't on the left hand side of an equals sign at any point like sourcetype was (which isn't a perfect rule, but usually works) so it's case insensitive. It'll match ERROR, error, Error, ERRor, errOr - anything with those 5 letters in a row, regardless of case.
Hope that helps, and happy Splunking!
-Rich
