Boundaries between CCM and DCM
The limit (or boundary) is due to discontinuous_conduction_mode not being able to supply enough power to the load and hence continuous_conduction_mode is entered and, when that happens (usually seamlessly), much more power can be supplied. 

Think about the inductance value, the maximum duty cycle and the switching frequency. 

In DCM, the inductor current linearly rises from zero to some value in the first half of the switching cycle. That linear rise is dictated by the incoming DC supply voltage, the inductance and the maximum duty cycle. The current reached determines the energy stored and that energy (when released to the output capacitor) has a limit because the inductor current (in DCM), begins from zero amps in each cycle.

For example, let's say the current rises to a maximum of 10 amps with 10 uH inductance during the first half of the switching cycle. The energy stored (and subsequently released) is \$LI^2/2\$ or 500 uJ. If the circuit switching rate is 100 kHz, the maximum power transferred in DCM is 50 watts (ignoring losses).

In CCM, the current can still linearly rise 10 amps in the same length of time (max duty cycle) but, imagine it starting at 10 amps and rose to 20 amps. The net energy released is now 2000 uJ - 500 uJ = 1500 uJ and this gives rise to a power of 150 watts at a switching frequency of 100 kHz.

So, the boundary is determined by input supply voltage, maximum duty cycle, inductance value and output power required.