Stata fits discrete choice models. Stata 15 will fit them with random coefficients. Discrete choice is another way of saying multinomial or conditional logistic regression. The word "mixed" is used by statisticians whenever some coefficients are random and others are fixed. Therefore, Stata 15 fits mixed logit models. These models are fit with the new
Random coefficients arise for many reasons, but there is a special reason researchers analyzing discrete choices might be interested in them. Random coefficients are a way around the IIA assumption. If you have a choice among walking, public transportation, or a car and you choose walking, the other two alternatives are irrelevant. Take one of them away, and you would still choose walking. Human beings sometimes violate this assumption, at least judged by their behavior.
Mathematically speaking, IIA makes alternatives independent after conditioning on covariates. If IIA is violated, then the alternatives would be correlated. Random coefficients allow that.