The governing principle of micrometer accuracy is that the reading is the true gap only if nothing foreign sits between the two contact surfaces that grip the work.

A micrometer measures by closing the spindle against the fixed anvil; the workpiece is trapped between these two polished surfaces, called the measuring faces.

Any dirt, oil, or metal chip on a measuring face adds its own thickness to the gap, so the instrument reads over the debris and gives a dimension larger than reality.

The faces also make hard metal-to-metal contact, so trapped grit can score or corrode them and permanently spoil the reference surfaces.

For both reasons the measuring faces are the part wiped clean after every use, typically with a clean cloth or paper drawn through the closed jaws.

The lock nut only clamps the spindle to hold a reading, so its cleanliness does not enter the measurement.

The ratchet stop and thimble are used to apply and set measuring pressure but never touch the workpiece, so debris on them cannot alter the dimension.

Hence, the answer is Measuring faces.