Atmos allows for arbitrary key/value pairs to be added to managed objects.  Custom HTTP headers are used to send these key/value pairs to Atmos.  When overlapping key/value pairs exist they will default to a listable metadata type.

Example:

x-emc-listable-meta: first=value1, second=value2, third=value3

x-emc-meta: first=value1

The key/value pair of first=value1 will default to listable in this condition.