All registerations/deregisterations are sorted by physical connector string.
Span registration should generally always succeed. Span unregistration may fail if channels from the span are in use by e.g. asterisk. In such a case you'll also see those channels as '(In use)' in the output of lszaptel(8).
on --- registers all XPD's to zaptel.
$ zt_registration
XBUS-02 [] usb-0000:00:1d.7-4
XBUS-00/XPD-00: on Span 1
XBUS-00/XPD-10: on Span 2
XBUS-00 [usb:00000126] usb-0000:00:1d.7-2
XBUS-02/XPD-00: on Span 3
XBUS-02/XPD-10: on Span 4
XBUS-02/XPD-20: on Span 5
XBUS-02/XPD-30: on Span 6
XBUS-01 [usb:00000128] usb-0000:00:1d.7-1
XBUS-01/XPD-00: on Span 7
XBUS-01/XPD-10: on Span 8
XBUS-01/XPD-20: on Span 9
XBUS-01/XPD-30: on Span 10
This should allow you to register / unregister a specific XPD rather than all of them.