Hi Lally,
The minimum of value models is zero. But then you cannot have a value or depreciations on your asset.
Within my country we usually setup only one value model just to write off.
Some countries have tax regulations where a part of e.g. the sales tax should be depreciated as well. Then you can setup the second.
I also had a scenario where one building had several parts of reporting. They used four value models. E.g. Original acquisition, revaluation, maintenance. These value models were setup separately, so maintenance did have depreciations and revaluation not.
The depreciation books are used for reporting purposes where the ledger will not be affected. SO you can use it e.g. for taxes, corporate reporting or other scenario's where you just want to know the calculated amounts without posting to your ledger. This option was introduced for the US and Canadian market.