The Tektronix TDS430A is a 400 MHz 100 MS/s oscilloscope. I know other ocsilloscopes which could be toggled between analog mode and digital mode, so that these numbers would make sense. However, it seems the TDS430A is digital only. How could they possibly measure 400 MHz with only 100 MS/s?
They're stating that the bandwidth of the analog front end is 400MHz. For a periodic signal then you could fill in the waveform by sampling successive sweeps.
So you could for example see a 400Mhz sine wave on the scope (though 3dB down) with this technique.
For a single shot or non-periodic signal you would be limited by the sample rate.
From the manual: