In ad hoc arbitration, the parties and tribunal manage the entire process themselves — appointments, procedural timetables, venue logistics and record-keeping — with no administering body beyond what the Act itself provides as a fallback.
Institutional arbitration places that administration with a body like an arbitration centre, which applies its own procedural rules, manages appointments and timelines, maintains the official record, and provides hearing infrastructure.
Ad hoc arbitration can work well between sophisticated, cooperative parties who want lower administrative cost. It tends to struggle when one party is uncooperative, since there is no institution to enforce deadlines or apply a fallback appointment mechanism — exactly the situations institutional administration is designed to handle.