Character you use to group keys.

C# |
public string Delimiter { get; set; }

All keys that contain the same string between the prefix,
if specified, and the first occurrence of the delimiter
after the prefix are grouped under a single result element,
CommonPrefixes. If you don't specify the prefix parameter,
then the substring starts at the beginning of the key. The
keys that are grouped under CommonPrefixes result element
are not returned elsewhere in the response.