The goal is to minimize the weekend training hours. Since a full training includes 3 training days we already can exclude the column for first training day from our objective function, since it is not possible to start on Saturday or Sunday. What we have left is x_i6 and x_i7, where i = the different companies and 6 and 7 = day 6 and 7 of a week (Saturday, Sunday). Now we simply include all training hours from all possible companies on training day 2 and 3 in our objective function. We need to include every company 3 times (6x_A6 = second training day on Saturday.
3x_A6 = third training day on Saturday
3x_A6 = third training day on Sunday)
(credits to Max :))