Technical trees are trees that could come down on something of value.  Such as: a house, cabin, out building, power line, valuable landscaping, public roadways, hot tubs, valuable immovable objects, and so on.  One way to define these trees is trees that must drop in a certain place, as opposed to being able to drop them where they want to go (i.e. direction they are leaning).  These trees tend to come down slower and often in pieces.  There is a higher rate for these trees, due to the skill and precision it takes to get them down safely.  They are listed on my invoices as hazard trees, because, doing it wrong could be hazardous to my insurance.