Search Replace Transfer
Transfer copies the contents of one field to another and gives you the choice of overwriting the existing text in the Copy To field, or adding to the Copy From and Copy To fields. This type of activity can be very specific to whatever you're trying to achieve, so I'll try to explain with a few examples
Here I am adding Exposure, the "Source field" to the "Target field", Caption. The Method is set to "Add after", meaning the Caption isn't replaced. I also added a line break between the Caption and the Exposure.
In the Preview area, I can use the slider to run through all the selected photos and review the results. When I press the "Transfer between fields" button, the transfer is processed.
Example 2 - Path to Keywords
Sometimes people have folder structures that they want to convert into keywords, and the plugin makes this relatively easy. When Path is selected as the source and Keywords as the target, the plugin converts each folder into a keyword.
In this case the photos already have keywords "Green" and "Patterns", and choosing "Add after" adds the folder keywords to the existing keywords.