Couchbase server identifies each update performed on the data with ever increasing sequence numbers. These sequence numbers are shared across multiple collections of the same bucket. Index service uses these sequence numbers to calculate “remaining mutations”. For example, if the high seq no at data service is 1000, and if the index is updated upto seq no 300, then index service will show 700 remaining mutations. But not all 700 mutations need to be transferred across services. Only those mutations will be transferred to index service which belong to the collection on which index is being built.
Hope this helps.