A named snapshot is the snapshot we create for backup and deployment purposes, it has a identifier assigned to it which is visible in the snapshot history.
Everytime a Process Designer User saves his changes an unnamed snapshot is created in Process Center, thus for a heavily loaded concurrent development Process Center environment it is possible to accumulate thousands of unnamed snapshots daily, bloating up the Process Center database and affecting performance.
You can setup automatic deletion of these snapshots please see the following link for detailed information
http://www.ibm.com/support/knowledgecenter/SSFTBX_8.5.5/com.ibm.wbpm.admin.doc/topics/managing_snapshots_j.html