A covered dish on a crowded table can leave the host guessing whether it should be hot, cold, mixed, or assembled. A short note prevents that uncertainty.
Write the dish name, ingredients, and any necessary serving instructions. If the dressing is separate, say so. If food needs reheating, arrange that with the host beforehand and give clear, appropriate instructions for the actual dish.
Bring the right utensil and a container that can be handled safely. Label your equipment if you want it returned. Avoid asking the host to find extra bowls, oven space, and a knife at the busiest moment.
For dietary requirements, be accurate rather than expansive. Say what the dish contains and how it was prepared, and do not label it suitable for every allergy because it happens to be vegan. Specific information is easier to trust.