从MSDN: Visual Basic Concepts开始：

Classes differ from standard modules in the way their data is stored.
There is never more than one copy of a standard module’s data. This
means that when one part of your program changes a public variable in
a standard module,and another part of your program subsequently reads
that variable,it will get the same value.

Class module data,on the other hand,exists separately for each
instance of the class.

从Devx.com: Class Module(.cls) vs. Module(.bas)开始：

Deciding between a standard module and a class module is not a
decision based on performance,but one of design. The main difference
between the two is in the way that they handle data. A standard module
stores only one copy of the data. A class module encapsulates the data
within each instance of the class. That is,for each instance of the
class,the data exists separately.

The other main difference is the scope of variables and procedures
within the module. In general,any variables and procedures declared
as Public within a standard module are visible anywhere in the project
or external programs if the standard module is in a component.
Variables and procedures declared as Public within a class module can
only be seen through a reference to an instance of the class module.

The lifetime of data and procedures stored within a module is affected
by which type of module is used. The lifetime of the data and
procedures in a class module is defined by the lifetime of the object.
So data and procedures are available only if a reference to the object
exists. Data and procedures declared within standard modules are
available for the lifetime of the program.

Therefore,to answer your question,if you are writing a function that you want available throughout the lifetime of the program and visible to all code in the application,then place the function within a standard module.