auxiliaryData: pointer to 4 integers. auxiliaryData should be a unique identifier different from 0. Use a large, random identifier, or better, your V-REP's serial number if the message is yours. Otherwise, use the identifier of some other module. auxiliaryData could be the messageID of the message you wish to send to another module. auxiliaryData and auxiliaryData can be any values specific to your application.
customData: customData of your application (the broadcaster is in charge to release that buffer). Can be NULL.
replyData: pointer to 4 integers that can be used by a module to reply to a broadcasted message. Can be NULL. If not NULL, all 4 values are automatically initialized to -1.
Broadcasted messages can be intercepted in a plugin's "v_repMessage"-function. In the function, broadcasted messages can be recognized when the function's first argument ("message") is sim_message_module_broadcast.