The excluded business definition is an exclusion from the tax on split income, or TOSI, and is intended to provide a bit more of bright-line exclusion than you have in the base reasonableness test. It applies where an individual makes significant labour contributions in support of a business, and if you work at least 20 hours per week during the portion of the year that that business is being carried on, as in a farming business, for example, that only operates through a portion of the year, then that test would only have to be met through that portion of the year, not the full 52 weeks.
That test is intended to provide an automatic exclusion from the tax on split income where you've made a significant labour contribution in support of a business. You can think of the excluded share concept as being the other side of that coin.
The reasonableness test just in general looks to your contributions of labour and capital in support of a business. The excluded business is a bright-line test for labour, and the excluded share test is a bright-line test that applies in respect of capital. So if you have a sufficient interest—more than 10%—in a corporation carrying on a business and that's determined by both votes and value, and that business earns less than 90% of its income from the provision of services, then you can be excluded from the tax on split income without having to go back and look to the reasonableness test. So it provides a much clearer bright-line test that is intended to ease compliance concerns for affected taxpayers.
Now you've asked why some businesses are affected and others are not. One of the questions we've received relates to the services business criterion, and so I could speak briefly on that if it's what you were thinking of.