This read-only enumerated value selects which of the ComPort's three software images - the vxWorksImage operating system, the applicationStu image, or a downloadImage held in DRAM - a given code-image row describes. An admin needs this to correctly interpret the accompanying version, build directory, and date/time fields, since they mean different things depending on whether they describe the OS, the application, or a pending downloaded image. For example, when auditing firmware versions fleet-wide, an operator filters on stuCodeImageIndex equal to applicationStu specifically to compare application software versions rather than accidentally comparing OS versions.