No single obvious place to put this note, but I don't remember ever seeing it made clear:

Master page names take the form: <masterpage>: (left|center|right) with the <masterpage>: part defaulting to 'default:'

Examples
'left'
'my_masterpage:right'

Scripting with PageStream

PageStream provides for both internal scripts (macros) and external scripts (Python and, on the Amiga, ARexx). What is scripting good for and which to use?

What is it for?

Scripts are generally used in one of two ways: to ease repetitive tasks or to extend the program.

Internal Scripts (Macros)

If you find yourself doing the same things over and over again you have found a good candidate for recording a macro. Start recording, do what you need to do, stop recording, name the macro and to get those same actions all you need to do is play it.

External Scripts (Python)

External scripts give the possibility of making PageStream do things that are tedious or not possible through the normal graphical interface. If you don't like the font selection methods provided you can write your own. Or provide a unique interface for designing specialized documents, such as calendars.

Python is the cross-platform language to use for writing external scripts, but on the Amiga platform the ARexx language can be used. Due to the extensive ARexx support on that platform it may make sense to use ARexx rather than Python. However, doing so precludes the script working on any other platform -- the script would need to be translated into Python in that case.