In the event the contesting parties or organizations are unable to agree on the employees eligible to participate in the selection of representatives, and either party makes application by letter for a formal hearing before the Board to determine the dispute, the Board may in its discretion hold a public hearing, at which all parties interested may present their contentions and argument, and at which the carrier concerned is usually invited to present factual information. At the conclusion of such hearings the Board customarily invites all interested parties to submit briefs supporting their views, and after considering the evidence and briefs, the Board makes a determination or finding, specifying the craft or class of employees eligible to participate in the designation of representatives.