Nemo judex in causa sua
no one should be a judge in their own cause
Where the phrase comes from
A rule of natural justice. Dr Bonham's Case (1610) is the historical English citation people reach for. The modern rule is recusal and independent decision where an interest is real.
What lawyers are pointing at
A person who stands to gain or lose by a decision should not be the one who makes it. The rule is about the interest, not about expertise. A doctor may decide a medical question. A doctor should not decide a complaint about their own fee. Identifying the interest and handing the decision to someone without it is the whole practice. Pretending there is no interest because the decision-maker is confident is the breach.
The same shape in code
An author who is the only approver of their own production change is judging their own cause. A required review from someone who cannot edit the branch, or a check the author cannot disable, is the separation. A self-approval button with a Latin motto under it does not fix the interest.
A concrete picture
A licensing board includes the owner of the only company that would lose if a new license is granted. That owner does not sit on that vote.