square box on trace

This occurs when that trace segment was placed by the autorouter. When you delete the cline attached to one side the remnant will stay there. It has to do with a fanout property on the cline.

The boxes can also be from a group property on the Cline that was cut or joined by routing from the other end of the net. Great for taking measurements along a net.

Do you have scheduled nets?
That box is a T point.

Regards,
BillZ
EMA Design automation

Hmmm ....

Also see this in our designs, but can't really nail it down to an individual cause - appears several possibilities from the responses.

Is there anyone from Cadence on the forum who can elaborate? (What causes them, why they are shown, what can we use them for, how we get rid of them ..)

In the help under net schedule it explains what T points are and how to schedule them.
Specctra also has virtual T's

These T's are used in scheduled nets to control the length mostly. an example of this is a clock net.
A single trace is route to a point (T-point) RefDes is T1.1 then it braches from the T to the module. The length from the T to the modules all have to be the same length.
U1 to T1 length equals 1000 mils
T1 to U2 length equals 500 mils
T1 to U3 length equals 500 mils
T1 to U4 length equals 500mils

Usually these T's are their for a reason. You can use a via as a T-point.
To verify this do a show element on the net and see if there is a T1.1 connections and if the net is scheduled.

Hope this helps
Regards,
BillZ
EMA Design Automation