A transformer can have multiple taps on its windings, allowing the input and output voltages to be adjusted to different levels. These taps are typically located at different points along the winding, and by connecting the appropriate tap, the turns ratio can be altered, resulting in different voltage ratios between the primary and secondary sides. This feature is often used for voltage regulation, especially in power distribution systems.
If “multi-tap transformer” has taken on a specific meaning or application beyond what I’ve described, I recommend checking more recent sources or documentation to get accurate and up-to-date information.
Heating element transformers are multi tap transformers to compensate for the increase of resistance of the heating element over time.