During the code generation phase ASPC does one very important task - symbols linking. A symbol is any identifier used in the statements in the source code. For example a variable used in statement is a symbol or function called is a symbol too.

Thus symbol represents a data or code unit of the program structure. Everything defined by the developer or defined as par of the standard language functions is a symbol. Execution of your code is calculation made using symbols (obtaining their value if they are variables or calling them if they are routines). Thus symbols are the most important thing in the application. 

The symbol selection mechanism depends on the general idea realized by the programming language or development tool. Thus VBScript differs from VB. Read Execution contexts to learn more about these differences and learn the details you need to know when using ASPC.

On the diagram you can see the order in which a symbol is searched. You can read the following description now or after reading the Execution contexts chapter. 

1. When processing a statement ASPC finds a symbol - a variable name or a function/Sub/Property name. It needs to know where this symbol points.
2. To learn that ASPC searches the containing routine. Remember that it is already analyzed and the information about the local variables and arguments is stored in the enveloping objects in the code structure tree. Thus ASPC looks if this symbol is not a local variable or routine argument. If so linking ends here and symbol remains in VB code "as is". If the symbol is not found there ASPC searches the node which contains the routine - it can be a VBS class or the Root class (class created from the global part of the code in your page/script). Then for example if symbol is found in the Root class ASPC changes the symbol name to point to the member of this class. To do so ASPC adds a member variable in the VBS classes - which points to the Root class. Thus all the created objects have "knowledge" for their creator and symbol change will include usage of this variable.
3. If not found there ASPC looks in the list of variables, constants and  routines declared DLLGlobal.
4. After all if the symbol is not found in the tree ASPC looks for it in the symbols file which lists the VB language standard Functions/Subs/Constants etc. And if found it changes the symbol as described in the file or places it "as is" - what is appropriate.
5. If the symbol is not found nor in the tree nor in the symbols file compiler issues a warning message to remind you that the symbol is not known. You do not need Option explicit when working with ASPC but these warnings are equivalent to the errors issued by VB with this option. If may want to ignore them but it is strongly recommended to declare all the variables to prevent at least future problems caused by their incorrect usage. Anyway VB seems to be the only popular compiled language which allows variables to not be declared. It is common rule across the compiled languages that variable declarations help to explicitly define the variables scope thus leaving the language to choose the scope may cause wrong selection (the compiler may not assume default scope as you think or you may forget what the variable is intended for).