The useful life for a class of assets is intended to be roughly a weighted average of the assets that are included there. We typically define our assets by the asset itself, rather than by the industry in which it's used. There would be an assumption that there should be some comparability to the lives of particular assets whether they're used in industry A or sector B. We would then try to get a sense of the representative life of that asset.
We acknowledge that this means the CCA rate applied to any particular asset may be too high or too low, depending on the actual experience for that particular asset. The way to compensate for that is through provisions related to recapture and terminal loss. When an asset does not depreciate as quickly as the CCA rate provides for and is then disposed of--let's say it sold for more than what it was written down for--then there is a potential that the difference will be taken back into income, reflecting that there were CCA deductions in excess of the depreciation actually realized.
On the other hand, when an asset depreciates much more rapidly than provided for, there can be a terminal loss, such that the additional deduction necessary to reflect the experience of that asset occurs at disposition.
The description I've given you becomes more complicated when you take into account that we group assets in pools. Some of these provisions would occur when a pool gets exhausted or if an asset is being depreciated on a stand-alone basis. The government introduced a provision several years back to try to make the system more reflective of differential experiences for actual depreciation by providing what we call a separate class election for manufacturing and processing equipment. If a business is concerned that an asset is going to depreciate more quickly than is reflected by the 30% rate provided for those assets, it can put it into a separate class. Then if it disposes of the asset after, say, four years, it can claim a terminal loss at that time, rather than allowing the difference to go into the general provisions for the pool.