This option is only available when you right click an attribute. Invocation copies the attribute's content tag to the clipboard and displays a message to this effect. On pasting the tag, it appears as per the following example (the GUID used will vary):
({Attribute;0020315f-085b-4233-a146-6906c8df65b3:Email Address})
The tag can be used when personalizing externally-authored RPI content (e.g. HTML asset or email offer). The content thus created can later be pasted into an RPI file, at which point the referenced attribute will be added as an embedded file automatically.