xbee and interoperability
Only the XBee products with "ZB" in the name provide "Interoperability with ZigBee devices from other vendors"

As I understand it:


the products with "ZB" in the name have the "Zigbee Feature Set" included in the device firmware. The code enables the device to use the Zigbee protocols needed to interoperate with Zigbee devices from other manufacturers.
People purchasing XBee "ZB" products from Digi do not have to pay a separate royalty payment to the Zigbee Alliance if the products are for their own use.
If you plan on incorporating Zigbee capable components into a product or service that you will be selling to other people, you need to read the legalese and that probably means
paying annual membership fees to the Zigbee Alliance


Caveat: I'm just some random person on the Internet, it usually isn't prudent to make legal or financial business decisions without paying a competent professional for advice.