XBEE API ARDUINO TO ARDUINO COMMUNICATION ?

Hi,
i#m a bit confused about the framework of the xbee protocol. i use two xbees one as a coordinator in api mode on a arduino mega and the others as router in at mode also connected to a arduino. I send data with variable payload. So everything works fine i work with the xbee-api library. But i dont understand the framing in detail even if i compare my output with the frame maker from digi i dont get it right. The offset for my payload dont match with anything i found on the internet. I would need a constant to identify the start of my payload. I analyze a 0x90 Receive Packet in API Mode 2 With a ZigBee device. Dose anybody know a website where i can find a detailed list with all the frame work for the xbee api ?

Thanks in advance.

The XBee modules API information can be found in the product manual for the XBee.