Field help (KALWCOD) 

Enter the code for the specific rule to be applied on asset depreciation for the schedule being processed. If field associations are set up for the Asset object, the Specific rule code can be automatically generated after entering the Accounting code or the Group code.
The rule to be applied is selected according to the depreciation mode selected and/or the depreciation schedule. 

For example, on the Fiscal plan, depreciated based on the French decreasing method, a specific rule can consists in applying a tax aid (see the Depreciation methods description documentation).
After validation, you can only change the value as follows:

Use the Change method from the Action panel for an asset or as a bulk action in the Change depreciation method function (FASMTCM ).

Use the the Change allocation method from the Action panel for an asset or as a bulk action in the Accounting allocation mod function (FASTRFCM).

Note on the 40% deduction [15/04/15-14/04/17]Specific rule for a French company:
To apply this rule, the purchase date must be on or before 14/04/17. If not, you receive the message "If the specific rule no longer applies at the purchase date, make sure you have the rights to apply it. Do you confirm?" You can select Yes or No to dismiss the message.
Yes: Applies the rule even if the purchase date is past 14/04/17.
No:   Creation or updates are not applied and you need to change the Specific rule to None or cancel the action.