Square waveform or pulse waveforms DC or AC?

When measuring square waveforms or pulse waveforms , sometimes on the oscilloscope mode I use AC to remove the DC offset, but other times I have to use the DC mode on the oscilloscope to see the square waveform or pulse waveforms, The AC mode doesn't show the square waveform or pulse waveform but other times it does work in the AC mode on the oscilloscope

So I'm confused as how does a tech know if his squarewaveform or pulse is AC or DC?

Is DC offset and DC component the same thing or is there a difference?

Sometimes when I'm using an oscilloscope the pulse train waveform has DC offset , so its 3 volts DC offset with a pulse train waveform riding on top of it, What is this called? when a squarewaveform, pulse or pulse train signal is riding on top of a DC offset or DC component?

When you use the AC mode, the oscilloscope places a capacitor in series with the input. This removes the DC component of the signal. Or in other words the mean value.
With a pure sine wave AC or DC mode has no effect (zero mean value). A square wave that goes from 0 to +10v (50% duty cycle) has a mean value of +5v. The AC mode will remove that +5v and it will shift down the waveform by that amount. If the same wave goes from -5v to +5 the mean value is 0 and again the AC/DC mode will not make difference.

In this context DC component and offset are the same thing.

So I'm confused as how does a tech know if his squarewaveform or pulse is AC or DC?

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being a little picky
DC is a constant value, no pulse. AC generally refers to alternating between + and -
https://en.wikipedia.org/wiki/Pulsating_direct_current
Many times how do you call the waveform is based in your application so it is just a point of view.
I think that is not important how you name it as long as you understand what are you measuring and what does it mean.

When you use the AC mode, the oscilloscope places a capacitor in series with the input. This removes the DC component of the signal.

Click to expand...

Yes I know, But in AC mode it just was a blank screen, no square waveform or pulse train at all

ONLY in DC mode, I could view the squarewaveform or pulse train waveform, it has DC offset of 3 volts and a squarewaveform/pulse train riding on top of it

DC is a constant value, no pulse. AC generally refers to alternating between + and -

Click to expand...

But it only works in DC mode not AC mode , why is that?

Is the squarewaveform/pulsetrain waveform is DC, alternating between from high to low than its AC or DC?

you need to re-adjust the triggering when you switch between modes (as the trigger level will change - depending on your scope)

My oscilloscope has autosetup button so it does it for me

If the waveform disappears from the screen when you switch AC/DC it can only be:
1.No triggering as AnnaConda said. Try to set it manually. Or switch NORMAL/AUTO triggering. Oscilloscopes used to have a led indicating that triggering is valid.
2. The waveform jumps out of view because of the vertical position control. Tektronix have a beam finder button to check this.
3. The oscilloscope is malfunctioning.

Is a DC levels that alternating from 0 to + 5 volts which is a square waveform, is this AC waveform or DC?

Because I tried changing the triggering and i switched to normal , but the waveform only displays in DC mode not AC mode

DC level is when you measure a battery. If there are pulses is AC for the oscilloscope. Although it not may be the exact definition of AC:
There is a AC/DC setting in the vertical input channel. Already discussed at my first post.
And a AC/DC setting of the triggering. That is the coupling of the triggering circuit.
A square wave should display in either vertical mode when triggering is correct. The only difference being the vertical position.
In normal mode of triggering you will not get any display when there is no trigger. Auto should display always.
What oscilloscope are you using?

Why does the triggering level change when you switch AC mode to DC mode?

A square waveform should display in AC mode and DC mode right?