Costs incurred to increase the operating efficiency or useful life of a plant asset are referred to as

Additions and improvements

Explanation:

Additions and improvements to the value of the assets are basically incurred to increase the life or usefulness of the assets which increases the productiveness of the plant or machinery. These expenditure on the fixed assets are considered as the capital expenditures.

Most of the firms incurred these costs to improve the operating efficiency of the assets and its productive capacity.