This column appears if the setting Use time codes for operations? has been configured under the heading Preparation that you will find under the Manufacturing tab in the Settings procedure. This is where you select a time code (TC).
The time code describes the basis of the operation times, for example if they are calculated using the Time Calculation you select Time Calculation. The option Undefined is set by default. You can also select your own time codes, which must first be defined in the Time Codes procedure in the Global Settings module.