Why does an analog oscilloscope have a frequency number written on it?
Probably the single most important parameter that tells you whether a particular scope is suitable for a particular job is its analog bandwidth. Signals that fall within this bandwidth will be displayed properly, while signals that exceed this bandwidth (even if just in their harmonics) will be distorted.

Since the bandwidth of (nearly) all scopes extends to DC, the important parameter is the upper frequency limit, and this is the number that is printed on the scope. This is the frequency at which the height of a sinewave display is reduced by 3 dB, or to about 70% of its nominal (low-frequency) value.