Depr key error

We changed the depreciation key for book depreciation and the new key depreciates to zero over the remaining useful life.X120-uses base method 0011 :1:17 and 1 for multilevel method

When posting manual depreciation we are getting the below error

manual depreciation not allowed with depreciation key X120

Becuase its a forecasting depreciation key

But I am able to post unplanned depreciation with the same key.

In this context what is the difference between unplanned and manual depreciation.

why would the system allow unplanned but not manual depreciation

What should I do to use manual depreciation for this key