Polyimide labels are rated to withstand high process temperatures.
Although Polyimide labels are high temperature labels they are rated for 270°c for 5 minutes.
It would be more accurate to describe the polyimide labels as high temperature process labels as they are not designed to withstand high temperatures long term.

The main application for polyimide labels is for PCB barcode indentification, to be attached to the PCB before reflow.
High temperature labels are sometimes referred to as Kapton® labels.
Kapton® is a polyimide film developed by DuPont in the late 1960s, various high temperature products are made from Kapton®, Kapton® tape, Kapton® masking dots and Kapton® is sometimes used to insulate during processing or part of the design of a product.

Marking the PCB at the start of the process ensures traceability is possible for the whole manufacturing process, invaluable when using a reflow oven monitoring system such as the KIC RPI, as when the PCB enters the reflow oven a barcode scanner reads the PCB barcode label, the KIC RPI software records the reflow oven temperatures that the PCB experiences and then provides full traceability of the reflow process for each board.

In most modern production lines the PCB barcode label is scanned at each stage of the manufacturing process, linking production builds to batches of components, solder paste/printing, placement and critically AOI, manual PCB Inspection and rework processes.

This information can then be looked up at a later date for traceability purposes.