You need evidence to prove or defend your case. Read Gathering Your Evidence about collecting and disclosing your evidence, and have it ready for the hearing. Evidence is primarily the verbal testimony of the parties and their witnesses. However, documents and records like leases, rent ledgers, communications, invoices, pictures, diagrams, and so forth, are very valuable to confirm verbal testimony of parties and their witnesses.
Preferably, all evidence should be given by a witness from their own personal observations or involvement. Little or no weight is usually given to "hearsay" evidence, if it is permitted at all. Letters, written reports and other information by persons who are not present to answer questions about what they saw or heard may not be allowed or may be given little or no weight. It is better that everyone with direct evidence that they saw or heard should be present or available by telephone so they can tell their story directly, and be questioned about their story. However, the rules of evidence used in courts do not apply, and a hearing officer may consider "any oral or written testimony or any record or thing that the hearing officer considers to be credible and trustworthy and relevant to the dispute." So while direct evidence is preferable, hearsay may be considered. Be prepared to justify why it is credible, trustworthy and relevant.
Written statements and other "hearsay" evidence may be substantially bolstered by having the witness available to be questioned in person or by telephone. They may not actually have to testify if neither the other party nor the hearing officer have questions about the written statement.