There is a document available from CustomerSource, Support, Knowledge Base, Retail Management, Store Operations, Customization, ArticleID 869720. I'm not sure if this is the elusive Customization Guide as it is pretty brief but it has a number of documents available for download as well.
This documentation has a download explaining hooks which lists the possible parameters shown below. Before I had the document, I had no idea what the parameters were, except that parameter 1 meant I could pass the Session object. I actually thought it meant the number of parameters to pass and that different hooks had different parameter lists.
The possible values of Parameter are:
0: Default - DR SMS to maintain compatibility 1: Session Object - A pointer to QSRules.Session 2: Digital Receipt - SMS - XML string of DR (SMS DTD) 3: Digital Receipt - ARTS - XML string of DR (ARTS DTD) 4: Nothing - No parameter passed
Have fun.