A unitary business group is comprised of two or more U.S. persons that satisfy both a control test and one of two relationship tests. MCL 206.611(6).

There is no annual date on which members must meet the requirements of a unitary business group to be a unitary business group. Rather, a person becomes a member of a unitary business group whenever both the control test and one of the two relationship tests are met and it remains a member of that unitary business group so long as the control test and one of the two relationship tests continue to be met. Conversely, a member of a unitary business group that fails either the control test or the relationship tests may no longer be included in the unitary business group.

Any person that becomes a member of a unitary business group or ceases to be a member of a unitary business group during that member's tax year must file as part of the combined return for that portion of the member's tax year during which the member was part of the unitary business group. For example, Taxpayer ABC is a unitary business group comprised of three corporations: Corporation A, the designated member with a calendar tax year, and Corporations B and C with fiscal years ending March 31, and September 30, respectively. Taxpayer ABC's tax year is that of its designated member. Thus, Taxpayer ABC's tax year ends December 31, its annual return is due April 30, and that annual return must include the tax years of Corporations B and C ending March 31, and September 30, respectively. If Corporation C ceased to be a member of Taxpayer ABC on July 31, Corporation C must include October 1- July 31 on Taxpayer ABC's annual return, but file as a separate taxpayer - or as part of a new unitary taxpayer - for the period August 1 - September 30.