HP 54710A User's Reference Manual page 82

Acquisition Menu
Record Length
Figure 3-10 shows the same 256-point record and a 50-ns/div time base scale.
However, the sampling rate was set to automatic, so the oscilloscope picks a
sample rate of 500 MSa/s to avoid filling up the waveform record too fast and
displaying a waveform like figure 3-9. Because the waveform record is only
256 points, the oscilloscope selected a lower sample rate in order to optimize
the amount of data stored in memory to fill up the screen. Notice that more
data is displayed.
1
(256 Points) = 512 ns of data
⁄
MSa
500
s
Because the screen can display up to 500 ns of data, about 97.7 percent of
the waveform record is displayed on the screen. The arrow on the right side
of the memory bar indicates that the data in the waveform record that is not
presently displayed is to the right side of the screen.
Figure 3–10
Automatic sample rate and manual record length
3–17