Add to multi-value fields by default: When you select multiple files, the metadata pane displays
Multiple values for fields that contain different data. If you make changes to one of these fields, the new value will usually replace the existing value in all selected files. If this option is enabled, for fields such as tags (which can contain multiple values), the new values you input will be appended to the existing values for all selected files, rather than replacing them.
Note that even if this option is turned off, you can add new values by prefixing them with a + character (and remove values using a - character).