A form is a class so the same principles apply to that as with any other class; the member must be public or otherwise accessible within the caller scope, an instance member requires an object reference to be called, and a shared member must be called by only qualifying the type, for example reference.InstanceMethod() or ClassName.SharedMethod()
A programming element exist in VB called Module, within that all members are shared and are also type promoted, so calling a method from that can be done globally without qualifying type, for example ModuleMethod()