I am looking for documentation on the item object sometimes used in Maestro form rules.
What are the available properties and methods of this object, when are they available and how are they set?
In general, rules are called with the parameters (data, item, info), and you will find some information about that in the article Advanced Debugging of Maestro Forms#FormItems
The properties and methods of an item vary according to the specific type of item it is, but the properties are set through the Properties tab in the Maestro designer:
You can access the properties in a rule editor by using the rule helpers. Right-click on an item and select Insert Property Value:
Item properties are available once the form has loaded, so you can take it that item properties are always available in your rule code.
You will find it useful to use the Chrome developer tools to inspect item properties, as well as the other rule parameters, as described in the Advanced Debugging article. Another quick way to break at rule code is to put the 'debugger;' statement in your rule. As long as you have developer tools active, the debugger will kick in when the statement is executed. Then you can inspect the rule's data, item and info parameters.