What is annexation?

Annexation is the removal of one parcel of land from an unincorporated area and the addition of that land to an adjacent city. The more attractive pieces of land to be annexed are those with commercial development that may generate additional revenue for a city, without necessarily increasing the service burden (expenditures). This is commonly referred to as "cherry picking".

I've heard historic narratives of how Salt Lake City wanted to annex the Brickyard Complex and had to take enough of the surrounding residential area to meet the 105% rule.

The 105% rule is in place to evaluate the revenues of the "new" city versus the cost of services. This rule applies to either annexation or incorporation. If the addition of an area to an existing city or the incorporation of an area generates revenues that are greater than 105% of the proposed areas cost of services, a boundary change must be made or other modifications to reduce revenues.

Source:Utah Code Section 10-2-101, 106(5); 109(3).