The number of discrete phase points contained in the value is changed to 0111...1111, the phase accumulator will overflow after only 2 reference-clock cycles (the minimum required by Nyquist).
This relationship is found in the basic tuning equation for DDS architecture: changes in the output frequency.
DDS devices are not limited to purely sinusoidal outputs.
Figure 2 shows the square-, triangular-, and sinusoidal outputs available from an AD9833. The binary number in the frequency register provides the main input to the phase accumulator.
Since sampling theory dictates that at least two samples per cycle are required to reconstruct the output waveform, the maximum fundamental output frequency of a DDS is /2.