In plots, you can only have Focus, Filters, Gobo's, Optics, Unit, Wattage, Channel, Patch, and Unit in or around the fixture. You could always repurpose one of these. Otherwise, use a report with columns with channel/Unit numbers and purpose. In Capture 2024 this can be added to a plot, in Capture 2023 and before you can print this as a separate sheet.