In most kinds of litigation, including in the employment tribunal, there is something called the burden of proof. The party with the burden of proof is the one who is responsible for establishing a particular fact or set of facts.
For example, in unfair dismissal cases, it is for the claimant to prove that he is an employee; he has the burden of proof. The employer does not need to prove that a claimant is not an employee. But the employer does have to prove that he had a potentially fair reason for dismissal; there he has the burden of proof.
Don’t confuse the burden of proof with the standard of proof, which is about to what level of certainty something must be proved. For example, in criminal cases the standard is ‘satisfied so that you are sure’, also expressed as ‘beyond all reasonable doubt’. In almost all civil litigation, including in the employment tribunal, the standard is ‘on the balance of probabilities’. In other words, that something is more likely than not to be true.
The burden of proof can, occasionally, be important. But normally it isn’t.
Tribunals very rarely decide cases by saying ‘We just have no idea of what happened here. So since the burden of proof is on the claimant [or respondent] they failed to discharge it and they lose’. They are far more likely to have reached a fairly firm conclusion about what they think probably happened.
Things get a bit more difficult in discrimination cases, because of the reversed burden of proof and its associated complications. Even there, however, cases rarely turn on the precise burden of proof.
What this means in practice is that you should just try to persuade the tribunal of everything they need to be convinced of in order for you to win, regardless of the burden of proof. Don’t rely on the burden of proof, thinking ‘I don’t need to worry about this. The burden is on the respondent and I don’t think they can meet it’. Try just as hard to prove your case when the burden is on the other side, as you do when it is on you.