If you notice in the datasheet, for the weight it says that it is at the wrist.
Having said that, usually there isn't any standard calculation method for robotic arms. It is basically striking a balance between:
- maximizing the load (to satisfy marketing)
- minimizing the load (to make the legal team stop complaining about possible lawsuits).
There was one notable exception I have had the pleasure to encounter (it was not on a robotic arm, but on a Stewart platform) that takes the time/effort to provide software that calculates the allowable load at any given position in space. Probably there are more like that.