DC versus AC problem. Suppose your DC power supply is set to 15 V and the vertical setting on the oscilloscope is at 5 V/div.

How many divisions will the trace deflect if you connect the output of the power supply to the scope with the "A input" switch set on DC?

How many divisions if the switch is on AC?

DC = 3 div. AC = 0

Explanation:

When the input is directed to the input circuitry, and the "A input" switch is set to DC, the pure 15 V DC signal will be showed on the screen, so, if the vertical setting is at 5 V / div, the trace will deflect exactly 3 div.

If the switch is on AC, as this setting inserts a capacitor in series (which is located here to block any unwanted DC component superimposed to an AC signal) the DC signal will be blocked, so no trace will be deflected on the screen (after completed the transient period).