Speaking from my experience, most devices indicating they are being powered by "passive PoE" means that the device does not support standards based PoE (802.3af or 802.3at). While standards based devices "negotiate" power needs, a passive device simply expects the power to be automatically provided.
You may be required to use the vendor's power injectors and/or switch hardware to provide power to those devices. You may or may not be able to use a standards based PoE power source that can be configured to provide power without negotiation.
End devices that are compliant with 802.3af should work with any device that supplies 802.3af or 802.3at power, even if from a different vendor. End devices that are compliant with 802.3at should work with any device that supplies 802.3at and may work with devices that supply 802.3af depending on they need the extra power that 802.3at can provide and/or if they can operate in a lower power mode on 802.3af.
If you have a choice, always go with a devices that supports standards based PoE. It gives you more options due to the interoperability between devices of different vendors.