How is General Purpose IO line pass turned on? Or associated bit to bit?

Yes, I suppose “General Purpose IO line pass” is what I’m asking about, but the question is . . . what AT command turns it on/off, and what associates which Dn on the input to a Dn on the output? Or . . . is it always on in 802.15.4 devices and always same=same on which Dn is mapped to which Dn? There are examples here http://www.digi.com/blog/videos/xbee-tech-tip-digital-io-line-passing-with-xbee/ and here http://examples.digi.com/sensors/802-15-4-digital-input-with-a-button/ But neither of them (or anywhere else I can find) explain what triggers the association from side to side. Or . . . is it just always on?

See http://knowledge.digi.com/articles/Knowledge_Base_Article/XBee-802-15-4-Digital-Input-Output-Line-Passing/?q=DIo+line+passing&l=en_US&fs=Search&pn=1

It walks you through the configuration.

It is the Addressing you use along with which lines you use. That is to say, DIO 0 on module one will go to DIO 0 on module 2.