What is an INTERLOCK in J1939?
General answer. Appears to relate. MA not be what's wanted. 

Interlocks are means of protection that do not allow some action to occur, or access to occur etc until or unless some condition is met.

Where there are a number of conditions which must be satisfied in time or mechanical sequence, each may require an interlock condition to b satisfied befoe the next stage can be initiated. Such sequential sets of interlock conditions can be called first interlock, second interlock etc indicating the order in which the conditions must be fulfilled.

"Interlock" does not seem to be a term which is used as a core part of J1939, but is used by some systems that implement J1939.

In some cases a subsytem may need to perform potentially dangerous task - eg a gear changing system may need to obtain throttle control to adjust engine speed during the gear changing sequence - but you would not usually want to allow the gear change system to have free control of the throttle. In such cases an interlock is initiated between gearchange and throttle systems for a limited period to provide the requisite functionality.

Here is a mechanical J1939 throttle control system. It provides hardware interlock on pin 3 with the rest of the system and will not function until interlock conditions have been met. In this case it is a manual control and the condition is the user knowing the "secret" rotation sequences requied to "trigger it".

J1939 electronic throttle with interlock





Also:

J1939 intro

and

J1939 overveiw - excellent references

J1939 implementations by manufacturer



General ; brake / shift interlock desirability mentioned here.

Ford electronic throttle module  -a number of mentions of interlock. It includes this table



FRC remote engine throttle - yellow lead mentioned in numerous diagrams is usually used for "pump engaged" inerlock.

LDSBBT see page 5 diagram. 

Here there are a number of interconnected interlocks.
 Their logical rules are not described and the diagram would require a little thinking to fully decipher but it can be seen that there are interlocks controlled by gearbox-in-neutral, PTO engaged, Park brake on and ignition, with outputs to Throttle interlock and Okay to pump interlock.