A notice of arbitration (or request for arbitration, under institutional rules) is the formal written communication by which a party invokes the arbitration clause and puts the other side on notice that a dispute is being referred to arbitration.
A well-drafted notice identifies the parties and the contract containing the arbitration agreement, states briefly the nature of the dispute and the relief sought, and — where the clause requires party-nominated arbitrators — names the claimant's nominee.
Under institutional administration, the registry logs the notice, confirms it meets the clause's formal requirements, and sets the clock running on the respondent's time to respond and nominate. Getting the notice right the first time avoids an early procedural objection that delays the tribunal's constitution.