When this check box is checked identifiers are case-sensitive, so for example the following are all different identifier (hi, Hi, hI, and HI).
When this check box is not checked, identifiers are not case-sensitive, so the identifiers in the previous example are all the same identifier.
Case-sensitive identifiers are disabled by default, since Pascal is normally not a case-sensitive language. If you enable this option then remember to use all lowercase for keywords and built-in identifiers (such as var, integer, input, writeln).