All of the keys (up to 1,000) that share the same prefix are grouped together. When counting the total numbers of returns by this API operation, this group of keys is considered as one item.
A response can contain CommonPrefixes only if you specify a delimiter.
CommonPrefixes contains all (if there are any) keys between Prefix and the next occurrence of the string specified by a delimiter.
CommonPrefixes lists keys that act like subdirectories in the directory specified by Prefix.
For example, if the prefix is notes/ and the delimiter is a slash (/) as in notes/summer/july, the common prefix is notes/summer/. All of the keys that roll up into a common prefix count as a single return when calculating the number of returns.