How to understand “ALTERNATIVE” and “PARALLEL” in multiple antenna ports readers?
The “ALTERNATIVE” and “PARALLEL” refer to how the reader treats its multiple antenna ports. 
In ALTERNATIVE, the reader activates one antenna port in one time according to the host’s command. All antenna ports can be seen as time discretely accessible. In PARALLEL, the reader uses signals from all antenna ports to decode tag at the same time. All antenna ports can be seen as concurrently accessible.
Readers with ALTERNATIVE multiple antenna ports is suitable for application needing more antennae but not requiring fast operation. It will cut down the reader costs in system implement.(e.g. item-level jewelry inventory). Readers with PARALLEL multiple antenna ports is favorable for application requiring 2D tag identification in channel RFID system.