Example:

Consider the Status attribute to define the progress of a task:

Attributes     =  Author, Task, Status
Options Status =  Start, Step1, Step2, End
Thread Display =  Task, Status

and logbook with three thread arranged like this:

|-- Task0, Start
|   |-- Task0, Step1
|   |-- Task0, Step2
|
|-- Task1, Start
|   |-- Task1, Step1
|
|-- Task2, Start


It is collapsed to:

+-- Task0, Step2
+-- Task1, Step1
|-- Task2, Start

Now, if you filter the Status using the "Start" value, you obtain again:

+-- Task0, Step2
+-- Task1, Step1
|-- Task2, Start

In my opinion, this is a bit confusing.

Using Step1 or Step2 values, nothing is selected (also confusing).