Appends are basically just extra voicebanks for different voice tones and moods; For example, you might have a cute append, or a whisper append, or a power append, etc. You just use them like any other voicebank, as they're usually standalone. However, you could also assign them to the prefix map to use multiple in the same project as well, kinda like if you were using a multipitch bank. (I think Gahata Meiji's set of vcv vbs work this way?)
The practice of making voicebank appends comes from the append banks Crypton started releasing for their Vocaloids during the V2 era. In addition to, say, the Miku vocal for example, you could purchase her append and gain access to six more vocals for her, each targeting a different mood and feel. Other Vocaloid companies have done this as well, such as the Extends for Internet Co. vocals.