Public

Indicates that the Sub procedure is accessible to all other procedures in all scripts.

Default

Used only with the Public keyword in a Class block to indicate that the Sub procedure is the default method for the class. An error occurs if more than one Default procedure is specified in a class.

Private

Indicates that the Sub procedure is accessible only to other procedures in the script where it is declared.

name

Name of the Sub; follows standard variable naming conventions.

arglist

List of variables representing arguments that are passed to the Sub procedure when it is called. Commas separate multiple variables.

statements

Any group of statements to be executed within the body of the Sub procedure.

The arglist argument has the following syntax and parts:

[ByVal | ByRef] varname[( )]

ByVal

Indicates that the argument is passed by value.

ByRef

Indicates that the argument is passed by reference.

varname

Name of the variable representing the argument; follows standard variable naming conventions.

If not explicitly specified using either Public or Private, Sub procedures are public by default, that is, they are visible to all other procedures in your script. The value of local variables in a Sub procedure is not preserved between calls to the procedure.

You can't define a Sub procedure inside any other procedure (e.g. Function or Property Get).

The Exit Sub statement causes an immediate exit from a Sub procedure. Program execution continues with the statement that follows the statement that called the Sub procedure. Any number of Exit Sub statements can appear anywhere in a Sub procedure.

Like a Function procedure, a Sub procedure is a separate procedure that can take arguments, perform a series of statements, and change the value of its arguments. However, unlike a Function procedure, which returns a value, a Sub procedure can't be used in an expression.

You call a Sub procedure using the procedure name followed by the argument list. See the Call statement for specific information on how to call Sub procedures.

Caution
Sub procedures can be recursive, that is, they can call themselves to perform a given task. However, recursion can lead to stack overflow.

Variables used in Sub procedures fall into two categories: those that are explicitly declared within the procedure and those that are not. Variables that are explicitly declared in a procedure (using Dim or the equivalent) are always local to the procedure. Variables that are used but not explicitly declared in a procedure are also local, unless they are explicitly declared at some higher level outside the procedure.

Caution

A procedure can use a variable that is not explicitly declared in the procedure, but a naming conflict can occur if anything you have defined at the script level has the same name. If your procedure refers to an undeclared variable that has the same name as another procedure, constant or variable, it is assumed that your procedure is referring to that script-level name. To avoid this kind of conflict, use an Option Explicit statement to force explicit declaration of variables.

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Reference