From The Foundry MODO SDK wiki
Visitors refers to the Visitor Interface which is used for enumerating containers or lists. This header defines three important classes.
- CLxUser_Visitor -- the user class for visitors, allowing them to be called from plug-in code.
- CLxImpl_AbstractVisitor -- implementation class that clients can inherit from to create their own visitors. Most user classes have Enum() methods that take these directly.
- CLxInst_OneVisitor -- if you really need a visitor COM object, this template allows you to create them.