If two people who are acquainted with one another eat together at the same table, and one eats meat while the other eats dairy, they are required to place a "Heker" – reminder – on the table. Since they are acquainted with one another, it is possible that they might share each other’s food, and will thus be eating meat with dairy. Therefore, they need to place in the front of them some object which is not normally on the table, which they will see and will remind them not to take each other’s food. If they do not have anything unusual to put on the table, they may write a note – like a "Post it" – reminding them not to take each other’s food. Placing this note on the table, somewhere whey can both see it, suffices to allow them to eat together at the same table.
It must be emphasized that this requirement applies only if the two people know each other. If they do not know each other, then a "Heker" is not needed, as people do not normally share their food with strangers.
The question was asked as to whether people traveling together require a "Heker" if they are eating meat and dairy next to each other on an airplane. One Rabbi claimed that although each passenger has his own tray, they might nevertheless require a "Heker" because the trays were made separately only for the purpose of maintaining order, and not with the intention of allowing the passengers to eat separately. As such, according to this line of reasoning, even though passengers eat on separate trays, we must view them as eating together at the same table, and thus if two acquaintances are seated next to each other and one eats meat while the other eats dairy, they must place a "Heker." However, this rationale seems very difficult to understand, as each passenger is given his own individual tray which is separate and apart from the trays next to him, and thus according to the accepted Halacha, passengers who are acquainted with one another do not require a "Heker." Even though they are eating very close to one another, and one eats meat while the other eats dairy, they do not need a "Heker." This is the ruling of Rav Moshe Shayo in his "Mehkereh Aretz."
Summary: If two people who know each other are eating at the same table, and one eats meat while the other eats dairy, they must place on the table an object which is normally on the table, or a note, to remind them not to share each other’s food. People traveling together on a plane do not require a reminder, since they each have a separate tray.