Any Catalog projects (i.e. anything that contains an XML spec) will go in the \bin\custom directory.

Client-side code does go in the \clientbin\custom directory.
These will be for Blackbaud AppFx client-side UI components. These will not contain XML specs but will contain .NET Windows Forms.

When we create a project in Visual Studio, we’ll typically choose Catalog Project or Client Component Project.
Catalog Projects go in \bin\custom and Client Component Projects go in \clientbin\custom.