Current content of a target group is exported. The export can only be fully processed (not incrementally).
Filename:
 [SENDER-ID]_targetgroup_[targetgroupID]_full.csv (respectively with individual date suffix)
Example:
"userId";"custID";"mail" "12345";"";"mail@mail.com"
If interested or for questions regarding the automatic export, please contact our support team via ticket. 
You find more export types in the article „Export: Which specifications has the batch process?“.