An offset arising from the light path through the prism glass, which must be set correctly in the instrument for the measured distance to be right. A mismatch puts every distance out by a fixed amount, often tens of millimetres - a systematic error that looks plausible and can invalidate a whole day's work.

No, and assuming so is a common and expensive error. Constants differ between manufacturers and between prism types from the same maker. When mixing equipment, confirm the constant for each prism and set it explicitly rather than relying on a default carried over from another job.

Because they are less thermally reactive than aluminium, so they move less as the sun heats one side during a long setup, and they damp vibration better. For precise control work that stability matters more than the weight penalty. Aluminium remains popular for general setting out where speed matters.

Through its bubble and its straightness. An out-of-adjustment circular bubble offsets every point by an amount that varies with how the pole is turned, and a bent pole cannot be compensated for at all. Both should be checked regularly, since a pole leads a hard life on site.

Mounting and precisely centring an instrument or target over a ground point, using an optical or laser plummet and its own levelling screws. It also allows forced centring, where instrument and target can be swapped in the same tribrach without disturbing the setup - which removes a significant source of error in traversing.

Bubbles and plummets should be checked regularly and after any knock or drop, because they go out of adjustment silently and corrupt every reading afterwards. It is quick to do in the field, and it is far more likely to be the source of an error than the instrument itself.

Robotic total station work, where the instrument tracks the prism automatically as the surveyor moves. Because it reflects from any direction, the pole does not need to be turned to face the instrument. Its accuracy is slightly lower than a single prism, so precise control work still uses a standard prism.