Planting medium is the soil or other substance you are considering to plant your tree in. Usually they specify the requirements. Sometimes they let you plant in rotting wood. Usually, if the rotting wood is still red but getting quite mushy and wet, that would be be a healthy enough environment in which your seedling can establish new roots (also depends on the general climate of the area, how much it rains over the year, and the beliefs of the forestry/checker). I’ve seen trees grow out of cracks in cliff faces, although I have to admit they were not very straight or marketable for the US housing industry. Generally, the original contractor wants the most bang for its buck and wants you to put their expensive seedlings into what would ensure them the greatest chance of survival. Obviously, soil is the most ideal, followed by black, mucky mushy stuff. But sometimes you have no choice and I’ve found myself sandwhiching poor seedlings between rocks (this particular contract condition was that it had to be at least three rocks, never two – unfortunately, a product of over-deforestation such that the excess water from lack of functional roots causes all the healthy soil to wash down the hillside into the river).
It is not your job to make sure every one of your trees survive, but whatever measure you care about your trees will be visible to the checker, meaning you will have fewer quality problems. The seedlings come equipped with juicy nutrients in their plug, to give them a bit of a head start. Usually the forestry likes to plant the trees during an optimal climatic season. You may find yourself planting in wet swamp which may become a dry desert within a month. It is a probabilities game and the industry must learn what is the best approach for each region. Your best approach is to develop a sense of where the best medium is, and aim for those spots. To get your number of required trees per plot, you can squeeze more in less desireable medium, if nothing better is available. You can push the limits according to what you think the forester is after, although sometimes the forester might prefer you did not plant a tree in a small area at all, if it is obvious the seedling would die anyway and it would be a waste of nursery resources and their budget. Otherwise, you may just need to go deeper into better medium. Sometimes there are limitations as to how deep you can plant your tree, or otherwise your own reasoning to avoid excessive work can be justified.
Fir tress generally like drier ground, spruce wetter, and pine can be more versatile, although generally it prefers drier ground too. Sometimes you may have to carry several species with you and the forester hopes you will fumble for the right tree for the right location. On the coast they might ask you to drop a little fertiliser package next to your planted root.
For me, since I like to be so conscientious, I feel I have developed a feel for what the tree needs and give it my best shot, even if it costs me money. When the ground is drier, especially if you choose a higher spot, I like to squeeze more of the surrounding soil or medium around the plug. Once it rains this can help retain a larger moist area around the tree and improve its chances of survival. When planting in rocks, I’d try to tap soil from surrounding areas so that it would fall down around the tree, but most ultra highballers would never dream of performing such nonsense. When closing your hole you can often use your shovel or your planting hand to draw extra soil from surrounding areas, if needed. If you plunge your shovel into a rotten log, instead of pulling it out to look for a better spot, you can plunge your shovel deeper to get to the juicy soil beneath it, and quickly flick away some of the rotten wood so as not to bury your tree, since you planted it so deep.
Basically, just imagine that the seedling needs some time to grow and establish roots, and that it needs to retain around it whatever amount of rainfall or ground water it can. The rest depends on what the forester is looking for, how much he/she will let you get away with, and what quality level you are shooting for.