I'm trying out the batch import feature for the first time and am confused by the creation of dc fields that end in [en_US].   

Here is an example, this being metadata dumped via metadata-export.    The first item was submitted through the xmlui interface.  The second was submitted via the batch 'import' command. 
id collection dc.date.issued dc.date.issued[en_US]
6 123456789/4 2012-08-27
10 123456789/4

I'm not sure why the second item has its issue date in a  dc.date.issued[en_US] field.   I only see the [en_US] notation on the field when batch editing metadata, and not when editing the item in the xmlui interface.  In the xmlui edit item interface it still says dc.date.issued. 

I did not use a language qualifier on the batch import dublin_core.xml, e.g. 

<dcvalue element="date" qualifier="issued">2011-01-26</dcvalue>

Is the [en_US] really a separate field?  Is it some artifact of batch importing and editing?  Any pointer where this is explained would be appreciated.