Constant growth rate vs sustainable growth rate

While doing practice questions I noticed questions where it says the sustainable growth rate, however in calculations there is no need to use Do(1+g)/(r-g). So, the question is, why in some cases we use D o /r to get the value and on other instances, we use D o**(1+g)/(r-g)**. Is the point in the difference between " sustainable growth rate" versus " constant growth rate" ?

Thanks

The sustainable growth rate satisfies:

g = ROE × b

The constant (Gordon) growth model is:

P0 = D0 (1 + g) / (rg) = D1 / (rg)

This is valid whether g is the sustainable growth rate or not. If g = 0, then,

P0 = D0 (1 + g) / (rg) = D0 (1 + 0) / (r − 0) = D0 / r = D1 / r