Meaning and use of " 4 default from company code and depreciation area"

Hello Guys,

I am creating a depreciation Key and would like to know the use and meaning of " 4 default from company code and depreciation area" while determining the scrap value in the assignement of calculation methods.

I used this value but got an error message "depreciation key not allowed" and I tried to use the dep.key in an asset master data. ( I precise that my key was created and activated in the right chart of depreciation).

On the contrary, When I use the other values ( 1, 2 or 3), It works perfectly...

My questions are

1- What is the meaning of " 4 default from company code and depreciation area"

2- Is there an additional customizing step to make in order to correctly use this option ?

THanks for your help