Merging multiple items into one lets you transfer metadata and attachments from multiple items into a single one. The items that have been merged are removed afterwards.
Multiple items can be merged where all metadata is copied into the target item. This can be done by selecting multiple items to merge via the multi selection tool as described here.
Then select some items (at least 2) and click the merge tool button.
This opens the merge view, where you can change the order of the selected items.
The order of this list can matter when merging (see below). You can move items via drag and drop or the buttons on the right.
Once you clicke the Merge button, the items are merged and you will be taken to the search view.
As you can see, tags are all combined. Custom fields of same name are also merged, where possible. For text fields, the values are concatenated with a comma as separator. Money and numeric fields are simply added together. Also it shows that there are now two attachments in the item.
Since the metadata of all items are merged into one, the order matters for fields that allow only one value (for example correspondents, concerning person/equipment, folder and dates). For these fields, the value of the first item in the list is used. The exception is the item notes: they are all concatenated with some newlines in between.
All properties that allow multiple values (like tags and the attachments, of course) are simply moved to the target item. Custom fields are merged depending on their type. Fields of type money and numeric are added together such that the final item contains the sum of all values. Text fields are concatenated using a comma as separator. Other fields (boolean and date) are again chosen from the first item that has a value.
After merging, the other items are removed from the database (they cannot be restored). This reason is that many data is moved into the target item and so the remaining items are actually empty.