If you instruct the VLISP compiler to link functions in your project directly, the compiler tries to resolve all explicit function calls by referencing the function's definition in memory. In contrast, when you indirectly link your functions, the compiler creates references to symbols that VLISP later uses to look up the actual memory location of the function. Direct linking improves the performance of the compiled code and protects the code against function redefinition. However, if your application needs to redefine a function, you cannot directly link that function.
Once function calls are directly linked, the compiler can optimize one level further by dropping the function name completely so that the function becomes invisible to users. To select this feature, choose the Internal Link mode option. Note that symbols exported to AutoCAD (for example, function names starting with C:) are never dropped.