/NOI[[GNORECASE]]
This option preserves case in identifiers. By default, LINK treats uppercase and lowercase letters as equivalent. Thus ABC, Abc, and abc are considered the same name. When you use the /NOI option, the linker distinguishes between uppercase and lowercase and considers these identifiers to be three different names.
In most high-level languages, identifiers are not case sensitive, so this option has no effect. However, case is significant in C. It's a good idea to use this option with C programs to catch misnamed identifiers.