Analog 4-20 mA Output module for Process Control
What Opto are saying is that the output of the sender is a 20 mA loop ....BUT...... It has an internal 24 VDC power source in the module.
These senders are used in single sensor loops, and by being internally powered they reduce your need for an external loop power supply. You cannot connect two of these sensor loops in series.

Read the datasheet for the device: http://www.opto22.com/site/documents/doc_drilldown.aspx?aid=1709  

Notice the difference in specification of the SNAP-AOA-3 which is both optically and transformer coupled output isolation, and does not supply loop current (so multiple sensors can be connected in series and it needs an external power supply) and the  SNAP-AOVA-8 which has an internal supply for the loop current.
You will also note that the analog voltage outputs of the  SNAP-AOVA-8 are all referenced to the same common....they are not individually isolated.