If you have multiple files selected, the drop down list for the highlighted field will list all of the current entries. This is helpful when you want to ensure you have a consistent tag. Then you could pick the one you really want and save the change.
Example;
Beatles
Beatles, The The Beatles
The Beetles
Your input will be saved into the selected files, not into the drop down list for future tagging steps.
The drop down boxes always shows the content of the current selected files (not the input from previous tagged files). If such a drop down box only show <blank> and <keep> then the current selected files have no values in this specific tag.
The tag panel's best use is to apply the same data to a lot of files. Data for individual files (like TITLE or TRACK) are best entered in the file list, or in respect to TRACK with the help of the track numbering assistant.
With the exception of multiline fields and the exception of the GENRE tag field, which shows a fixed selection list regardless of the content of the selected files.