What is Multistate Tax Commission?

Legal Definition
The Multistate Tax Commission (or MTC) is a United States intergovernmental state tax agency created by the Multistate Tax Compact in 1967. As of 2011, 47 states are members of the Commission in some capacity. Commission members, acting together, attempt to promote uniformity in state tax laws. Its actions do not have the force of law, and member states may opt not to follow its recommendations.

States require purchasers of goods who claim exemption from sales tax to provide certification to the seller of such exemption. The Commission has developed a Uniform Sales and Use Tax Exemption Certificate used by most states.

The Multistate Tax Compact provides that the MTC may conduct audits of taxpayers on behalf of those states specifically authorizing such action. The Compact also provides definitions of terms that may be used by states in writing their tax laws.
-- Wikipedia
Legal Definition
A tax agency working with multi-state / multi-national businesses to assist with any tax-related questions or concerns. he appropriate entities are insured that proper taxes are paid on-time.