Under the Minnesota Constitution, Article 4, Sec. 17, only single subject laws may be passed by the Legislature. Theoretically, this requires that only amendments directly related, or "germane" to the measure, be attached to a bill. (The term "garbage bill" is sometimes used when a bill contains what some people feel are unrelated subjects.)
When a bill is being amended in committee, the committee chair rules whether an amendment is germane; on the House floor, the Speaker of the House makes those decisions. In the Senate, the committee chair makes the same rulings, and such decisions are left up to the President of the Senate on the floor. At times the courts have been asked to rule on this matter.