The Singles Only rewalk type is rarely needed, and only in specific
scenarios.
Singles Only is like a refresh walk (doesn't create a new database), but
it skips all the normal walking like Base URLs and refreshing content in
the index. Instead it only walks "singles" settings (Single Pages, Single
URLs, and Single Files). Further, every URL from singles is checked on every
walk, regardless of whether it would be scheduled based on the refresh schedule
described earlier.
When to use Singles Only walks - Singles Only walks
can be useful in scenarios where customers want something more efficient than
refresh walking, like a dataload environment, but aren't able to construct
proper dataload requests. If customers can produce a "changelist" URL that
automatically lists all URLs that have changed recently, then that changelist
URL can be named as a Single URL. A Singles Only walk will walk those
URLs, without attempting to refresh the rest of the indexed content that the
customer knows hasn't changed.