...
Select the gear icon next to the attribute.
Select Configure in the dropdown.
Under the General tab, select Show sum.
Creating a Reference for Attributes (Connecting 2 Objects)
There are 2 types:
Outbound: current object to another object
Inbound: another object to current object
Select the heading.
Select Attributes.
...
In the listing of attributes, enter a Name and Description.
In the listing of attributes, under Type enter Object, and under Type Value select the object type you want to reference.
In the listing of attributes, under Additional Value, select the Reference Type of this reference.
Select Add.