What is the purpose of a slippage mark on an instrument glass?

A slippage mark on an instrument glass serves to indicate if the glass has moved from its original position. This is important because even a slight movement can affect the accuracy of the readings provided by the instrument. If the glass slips, it can lead to hazardous situations where pilots are not able to trust the displayed data.

Understanding the significance of slippage marks helps in effective maintenance and operation of flight instruments, ensuring that they remain reliable and safe. The other options touch on different aspects of instrument markings but do not pertain to the specific role of slippage marks in indicating the physical alignment of instrument glass.