Two different logics
Accounting depreciation reflects the economic consumption of the expected benefit of an asset. Depreciation allowed for tax purposes follows its own rules. When the two diverge, the difference must be recorded somewhere.
The role of the derogatory
This difference is not an additional economic burden. It is recorded as a regulated provision, which makes it possible to keep accounting depreciation faithful to reality while benefiting from tax treatment.
An allocation, then a recovery
The mechanism is temporary. An allocation appears as long as the tax depreciation exceeds the accounting depreciation; a recovery then occurs, when the relationship is reversed. Over the life of the asset, the whole thing is neutralized.
The impact on equity
The regulated provision appears in equity, not in debt. This explains why a company can display equity influenced by a tax choice, without its economic situation having changed.
The classic trap
Does not add the exceptional depreciation to the calculation of the net book value as if it were additional consumption. The VNC is based on accounting depreciation; the exemption follows the tax incidence, separately.