http://www.legislation.gov.uk/ukpga/2001/2/section/54/notesMineral extraction, mining, quarryingLegislationLegal servicesTaxCapital expenditureExplanatory Notes to Capital Allowances Act 2001Inland RevenueKing's Printer of Acts of Parliament2018-08-24 2 22nd March 2001 273 This section sets out explicitly the different types of pool. In CAA 1990 the class pools and single asset pools are notional trades. The main pool is the default for the actual trade – or the activity treated as a trade. 274 Subsection (2) provides that a single asset pool cannot contain expenditure in respect of more than one asset. CAA 1990 achieves this by creating a notional trade for expenditure on the provision of an asset separate from any other trade carried on. 275 Subsection (3) points to provisions which determine if qualifying expenditure has to be allocated to a single asset pool (if it is allocated to a pool). 276 Subsection (4) and (5) provide similarly for class pools. 277 Subsection (6) provides what is the default rule in CAA 1990. This is that expenditure can only be allocated to the main pool if it does not have to be allocated to a class or single asset pool. Figure 2: example of requirement that expenditure relating to the different activities must not be allocated to the same pool.