In response to the original question, the FAA categorize helicopter by two distinct groups, with variations within the groups.
FAR § 27.1 Applicability.
(a) This part prescribes airworthiness standards for the issue of type certificates, and changes to those certificates, for normal category rotorcraft with maximum weights of 7,000 pounds or less and nine or less passenger seats.
(b) Each person who applies under Part 21 for such a certificate or change must show compliance with the applicable requirements of this part.
(c) Multiengine rotorcraft may be type certified as Category A provided the requirements referenced in appendix C of this part are met.
FAR § 29.1 Applicability.
(a) This part prescribes airworthiness standards for the issue of type certificates, and changes to those certificates, for transport category rotorcraft.
(b) Transport category rotorcraft must be certificated in accordance with either the Category A or Category B requirements of this part. A multiengine rotorcraft may be type certificated as both Category A and Category B with appropriate and different operating limitations for each category.
(c) Rotorcraft with a maximum weight greater than 20,000 pounds and 10 or more passenger seats must be type certificated as Category A rotorcraft.
(d) Rotorcraft with a maximum weight greater than 20,000 pounds and nine or less passenger seats may be type certificated as Category B rotorcraft provided the Category A requirements of Subparts C, D, E, and F of this part are met.
(e) Rotorcraft with a maximum weight of 20,000 pounds or less but with 10 or more passenger seats may be type certificated as Category B rotorcraft provided the Category A requirements of §§29.67(a)(2), 29.87, 29.1517, and subparts C, D, E, and F of this part are met.
(f) Rotorcraft with a maximum weight of 20,000 pounds or less and nine or less passenger seats may be type certificated as Category B rotorcraft.
Additionally, helicopters over 12,500 pounds require pilots to obtain a type rating.
FAR § 61.31 Type rating requirements, additional training, and authorization requirements.
(a) Type ratings required. A person who acts as a pilot in command of any of the following aircraft must hold a type rating for that aircraft:
(1) Large aircraft (except lighter-than-air).
Note: Large aircraft means aircraft of more than 12,500 pounds, maximum certificated takeoff weight.
Part 135 further categorizes operations into 10 or more passengers (or 9 or less).
The real separation lies solely within the Forest Service criteria for contracting, as the FAA have nothing to do with the control of the operation.