You need to understand the concept of "scope". It is all about where the function (or variable or sub) is meant to be available within the Project.
A Private function (or Sub) within an Access Object Module is only available from within the module where it is located. It can be called using its name alone (plus any parameters). These functions are used to do jobs that don't need to be initiated from outside the object. The Event Procedures called by objects within the module are typical examples.
A Public function in an Access Object Module becomes a Method of the object and can be called using the object name and function as you have found. These are used when the Method needs to be called from outside the object.
A Public function in a Standard Module has a scope of the entire Project and can even be called from queries by simply using its name (and parameters if required). By default a function in a Standard module is Public if its scope not explicitly declared.
A Private function inside a Standard Module only has scope within the module. These are used when the function is only intended to support the public functions of that module.
For best practice, the scope should always be limited to the minimum requirement because it is far less likely to become confused with other functions which have been given the same name within other scopes. When a function is called, Access looks first at the scope of the call. If it finds a match then that version is used. If not then it will look to a Public function in a Standard module. This allows different objects to have the same function names and even for the the same function name to be used at the Project wide scope.
If the function is only required for use within particular object module then that is where it should be placed. It is completely inappropriate to place a general purpose Public function inside an object module and call it from outside. Only Methods of the object (ie things that apply only to that object) should be used in this way.
