Basically, the point is that you simply ignore rollover nights allotted to the current year (excess nights obtained over the qualifying threshold in the previous year) in determining how many rollover nights you will have for the subsequent year, since they can only be used once (rollover does not itself rollover).
So you simply had 45 nights, 5 more than Gold requires and so you will have 5 rollover nights for the next year.
In this example, the 5 rollover nights from 2018 don't matter, but if instead you managed only 35 nights in 2019 the rollover nights from 2018 would give you Gold for 2020. You'd obviously have zero rollover though - as your actual
#nights is still below the 40-night threshold (-5 gets set to zero).
However, if you had 55 stays in 2019 then the 5 rollover nights would give you Diamond, so in this case the rollover nights would matter also (and you have zero rollover).
So if you have N rollover nights from 2018, then from to 40 to 60-(N+1) nights in 2019 it would not affect your status for 2020 (you made enough for Gold anyway but the number of rollover nights was not enough to bump you up to Diamond). But from 40-N to 39 nights, and from 60-N to 59 nights, the rollover nights would make a difference.