On his website, interstate motor carrier operations expert witness Lew Grill provides links to the Federal Motor Carrier Safety Regulations, Single State Registration System:
§367.3 Selection of registration State.
(a) Each motor carrier required to register and pay filing fees must select a single participating State as its registration State. The carrier must select the State in which it maintains its principal place of business, if such State is a participating State. A carrier that maintains its principal place of business outside of a participating State must select the State in which it will operate the largest number of motor vehicles during the next registration year. In the event a carrier will operate the same largest number of vehicles in more than one State, it must select one of those States.
(b) A carrier may not change its registration State unless it changes its principal place of business or its registration State ceases participating in the program, in which case the carrier must select a registration State for the next registration year under the standards of paragraph (a) of this section.
(c) A carrier must give notice of its selection to the State commission of its selected registration State, and, the State commission of its prior registration State, within 30 days after it has made its selection. If a carrier changes its principal place of business during the annual registration period specified in §367.4(b)(2), the carrier may continue to use its prior registration State, if any, for the next registration year.
(d) A carrier must give notice of its selection to its insurer or insurers as soon as practicable after it has made its selection.
[58 FR 28933, May 18, 1993. Redesignated at 61 FR 54707, Oct. 21, 1996, as amended at 62 FR 15420, Apr. 1, 1997]
http://www.fmcsa.dot.gov/rules-regulations/administration/fmcsr/fmcsrguidedetails.aspx?rule_toc=737§ion_toc=737