How much you can learn due a Siglent oscilloscope service manual ?

[Kiriakos GR]

As always I was looking for something else, and I come across of a new discovery. 

Siglent oscilloscope service manual,  this is not actually a detailed repair manual, but this has a bunch of tips in it.
From the location of screws, down to accuracy verification tests, and even voltage and frequency test points.

I did fall in love with the chapter Verify DC Gain Accuracy 
This involves a DC calibrator ( I have it), along testing at several voltage /Div.
Lowest DC input, this starts from 3mV,  and this performance test, this also requires testing at reversed polarity ( my calibrator can do that too).

I have my own scope GDS-2102A for 13 yeas, and I never paid much of attention of how to measure DC with it.

Long story in sort,  SIGLENT helped me to discover the value of  MEAN automated measurement.
MEAN this is the only one measurement which displaying symbol of plus or minus, at the front of the measurement.

At the world of digital multimeter, the polarity indicator this is in front of your eyes all times.
At the world of oscilloscopes, this is somewhat hidden.

GW Instek, offered at GDS-2000A series for free the DMM AP.
This has the option DC or DC RMS.
I was using always DC RMS or AC RMS ....   and today I did discover that if I select DC only,  the AP this shown a measurement with polarity indicator.  

Within two hours of DC voltages testing, so to discover of how accurate is my scope, this task ended as a heavy duty seminar.
Now that I am writing these words, I am very calm and happy and tired.
DC accuracy of my scope this is as tight as 0,5%,  after running SPC Calibration + Vertical Accuracy Calibration, due dedicated CAL Output at the back of the scope.

For a first time I did notice, that SIGLENT this is not using a dedicated CAL Output BNC.
All SIGLENT scopes, they use some sort of internal path for calibration routines. 

Now I have a new question, that neither AI this can answer.
Which method delivers most trust worthy calibration results?
The internal ? Or the use of dedicated CAL Output?

Food for thought