Please refer to the following Primus:  emc186798: "How to determine if a PowerPath license key is required for a server attached to a CLARiiON storage system."  As you'll read, for the AX series, a license is not required when connected exclusively to this model and in nonvirtual environments (PowerPath/VE is sold separately).  I'm sure you've all seen the page during the PowerPath installation asking if this will be connected to an "AX" series; select "Yes".  Finally, for the entry level model for each family of CLARiiON: CX3-10 and CX4-120 you should have received a single PowerPath ("Physical" and not "Virtual" /VE) license which you are to allowed use for all hosts that are connected exclusively to your CX4-120.  It should have come bundled with the array and comes in an off-white cardboard 8.5 x 11" cardboard envelope with red letters informing you "Do not lose".  Inside will be the single license key with multiple sheets differing only in language stapled together.
I have a host connected to an AX4 and a CX4-120, within the powerpath console on the host the "Path Status" within the disks view is displaying "Unlicenced" for the LUN's on the CX4 but "Alive" on the LUN's connected to the AX4.   I understood that we do not not need to licence Powerpath against a CX4?
Powerpath Licencing