When "Source cluster (2)" indicates that a universe of the default indicated on the universe screen is developed, and " Source cluster(2)" is developed, it's indicated as follows.
[Filter] Not set Enterprise Team Center
1. What is indicated with the "Source cluster" set as a universe of default?
2. Which item are you pointing at with "Not set"?
How should it be done in the purpose which doesn't make this indicate?
When creating a new universe, some attributes that can be selected include "Not set" depending on the attribute when selecting an attribute list by selecting [Add Filter] and selecting an attribute value. (Type is not included.)
Select "Not set", click "Set" and create by clicking "Save", "Not set" is included in the pull down list.
Can edit  by action, if you deselect "not set" and click [Set] and [Save], "not set" will not be included.
In general, "Not Set" means the value (for an attribute) is null.
It might be easier if you just treat "Not Set" as "null", meaning the situation when a particular attribute has a value as null.
in another word, it's like an attribute has following possible values: x, y, z, null.
For example, if the attribute is Agent, only "Not set":
If a new Agent connects after the universe is created, a new Agent will appear in the experience.
In this situation "Not set" is used.