The {$S+} directive tells the compiler to generate stack checking code. This generates code to check if a stack overflow occurred, i.e. to see whether the stack has grown beyond its maximally allowed size. If the stack grows beyond the maximum size, then a run-time error is generated, and the program will exit with exit code 202.
Specifying {$S-} will turn generation of stack-checking code off.
The command line compiler switch -Ct has the same effect as the {$S+} directive.
By default, no stack checking is performed.
Remark: Stack checking can only be used to provide help during debugging, to try and track routines that use an excessive amount of local memory. It is not intended and cannot be used to actually safely handle such errors. It does not matter whether the error handling is through exception handling or otherwise.
When a stack error occurs, this is a fatal error and the application cannot be kept running correctly, neither in a production environment, nor under debugging.