What follows is an opinion from a United States Court of Appeals.
Intervenors who participated as parties at the courts of appeals should be counted as either appellants or respondents when it can be determined whose position they supported. For example, if there were two plaintiffs who lost in district court, appealed, and were joined by four intervenors who also asked the court of appeals to reverse the district court, the number of appellants should be coded as six.
When coding the detailed nature of participants, use your personal knowledge about the participants, if you are completely confident of the accuracy of your knowledge, even if the specific information is not in the opinion. For example, if "IBM" is listed as the appellant it could be classified as "clearly national or international in scope" even if the opinion did not indicate the scope of the business. 

Your task concerns the first listed appellant. The nature of this litigant falls into the category "private organization or association", specifically "business, trade, professional, or union (BTPU)". Your task is to determine what subcategory of private association best describes this litigant.

Opinion:
INTERNATIONAL UNION OF OPERATING ENGINEERS, LOCAL NO. 18, AFL-CIO, Plaintiff-Appellant, v. The DAYTON POWER & LIGHT COMPANY, Defendant-Appellee.
No. 73-1905.
United States Court of Appeals, Sixth Circuit.
Argued Feb. 15, 1974.
Decided July 18, 1974.
Jeffrey A. Belkin, Belkin & Belkin, Cleveland, Ohio, on brief, for plaintiff-appellant.
J. R. Newlin, Dayton, Ohio, on brief, for defendant-appellee.
Before PECK and ENGEL, Circuit Judges, and CONTIE , District Judge.
Honorable Leroy J. Contie, Jr., United States District Judge for the Northern District of Ohio, sitting by designation.
PER CURIAM.
Appellant union instituted this action against the Dayton Power & Light Company (hereinafter “Dayton”), seeking to compel Dayton to submit a grievance to binding arbitration. The grievance involved appellant’s allegation that Dayton had violated the terms of a “Memorandum of Understanding” between Dayton and local construction unions. This matter is before us on appeal from the district court’s denial of appellant’s motion for summary judgment and subsequent entry of an order of dismissal under Rule 56, Fed.R.Civ.P.
Dayton is one of three public utilities which own the J. M. Stuart Electric Generating Station on the Ohio River. Pursuant to agreements among the owners, Dayton has acted for the other two companies in connection with the construction of Stuart Station. The owners further delegated their authority by appointing the Maxon Construction Company as their agent and construction manager on the project. Maxon, in its capacity as agent, executed a “Memorandum of Understanding” with the South Central Ohio Building and Construction Trades Council, of which appellant union is a constituent member. The agreement, which covers all construction work at the project site, provides that in return for certain pay rate concessions the building trade unions will not strike. Dispute settlement is governed by the final clause of the agreement which in pertinent part.reads as follows: “It is agreed that any grievance between local union(s) and contractor(s) shall be settled in accordance with the grievance machinery provided in the craft agreement (s) or, if none is so provided, by arbitration.” It was Dayton’s refusal to process a grievance filed thereunder that led to the filing of this action. On appeal, appellant urges this Court to reverse the decision of the district court and to order that appellant’s grievance be submitted immediately to binding arbitration. For the reasons appearing hereinafter, we decline to so hold.
The sole issue here presented is whether or not the Memorandum obligated Dayton to arbitrate any dispute with the South Central Ohio Building and Construction Trades Council or any of its members. Of course, “[n]o obligation to arbitrate a labor dispute arises solely by operation of law. The law compels a party to submit his grievance to arbitration only if he has contracted to do so.” Gateway Coal Co. v. Mine Workers, 414 U.S. 368, 94 S.Ct. 629, 635, 38 L.Ed.2d 583 (1974). The question of whether a party has contracted to arbitrate is a matter for the courts to decide on the basis of the contract entered into by the parties. Atkinson v. Sinclair Refining Co., 370 U.S. 238, 82 S.Ct. 1318, 8 L.Ed.2d 462 (1962). In the instant case, although clearly a party to the Memorandum, Dayton was a party to it as an owner, not as a contractor. Dayton employed no craft union members at Stuart Station or otherwise and it had no labor-management contract with the appellant. As the district court pointed out, the dispute involved herein is “between Dayton and a labor union — between the owner and the union, as distinguished between [sic] the contractor and a union.” The arbitration clause does not constitute an agreement between the owner and the building trade unions working at Stuart Station to arbitrate disputes arising between these parties.
It thus appearing that the unambiguous language of the “Memorandum of Understanding” militates against the position asserted by appellant, we hold that the district court’s order dismissing the complaint was correctly entered, and the judgment in favor of Dayton is, therefore, affirmed.

Question: This question concerns the first listed appellant. The nature of this litigant falls into the category "private organization or association", specifically "business, trade, professional, or union (BTPU)". What subcategory of private association best describes this litigant?

Choices:
Business or trade association
utilities co-ops
Professional association - other than law or medicine
Legal professional association
Medical professional association
AFL-CIO union (private)
Other private union
Private Union - unable to determine whether in AFL-CIO
Public employee union- in AFL-CIO (include groups called professional organizations if their role includes bargaining over wages and work conditions)
Public Employee Union - not in AFL-CIO
Public Employee Union - unable to determine if in AFL-CIO
Union pension fund; other union funds (e.g., vacation funds)
Other
Unclear

Answer: 5