When you say anchoring point, I'm going to assume that you mean the point where the lanyard is attached. That said, I think the answer to your question is that the point load force created on the anchoring point as a result of a person falling (100kg multiplied by 'g' multiplied by the distance of the fall before the "arrest"). So the two and a half tonnes represents the strength required of the anchoring point so that it will not break when the force of a fall is placed upon it. Remember, as I said above this will be a point loading which focuses all the forces onto one point. In short, the anchoring point needs to be extremely strong to withstand the forces created by a falling person.