Short answer. Four categories are outside the exclusion order: a party who is a natural person, a designated representative of a juridical entity that is a party, a person whose presence is essential to presenting the party's cause, and a person a statute authorises to be present.
What the law says
This rule does not authorize exclusion of (a) a party who is a natural person, (b) a duly designated representative of a juridical entity which is a party to the case, (c) a person whose presence is essential to the presentation of the party's cause, or (d) a person authorized by a statute to be present.
Rule 132, Section 15 — Exclusion and separation of witnesses. Read the full provision →
Exclusion is the default, with four carve-outs
The court, on its own initiative or on motion, shall order witnesses excluded so that they cannot hear the testimony of other witnesses. The purpose is plain enough: a witness who has listened to the others can shape an account around them, whether deliberately or not. The rule then names who that order does not reach. The list is closed and specific, and it is the reason an exclusion order does not empty the courtroom of everyone who might later be called to the stand.
A party who is a natural person
An individual party stays, even if they will testify. That is not an oversight. A litigant has to be able to hear the evidence against them and instruct their counsel as it comes out, and excluding them would put them at a disadvantage that no witness faces. The carve-out is limited to a party who is a natural person, which is exactly why the next category is needed.
A representative of a corporation or other entity
A company cannot sit in a courtroom. So the rule allows a duly designated representative of a juridical entity which is a party to the case to remain. The word designated does work here: the entity is expected to nominate its representative, rather than rotating officers through the seat. In substance this gives an organisational party the same practical position that an individual party has as of right.
Essential presence, and statutory authority
The third category — a person whose presence is essential to the presentation of the party's cause — is the one that must be justified rather than asserted. It is aimed at someone the conduct of the case genuinely depends on, and the burden of showing that lies with the party seeking to keep them. The fourth simply preserves any statutory right to be present. Note also that the court may keep witnesses separate and prevent them from conversing, directly or through intermediaries, until all have been examined.