Read-only property of the ScriptLine: BookFormat.
The property "# Defined Pages" tells you the number of pages for which you have defined content. It is the total number of pages for all the ScriptLines you have defined.
If the value of this property is equal to that of the signature pages property, then that number is displayed alone (as shown above on the left). If the two values differ, then both are displayed with the number of defined pages appearing first (as shown below).
Note that when ScriptLines have a force start property set to a value different than "none", this can cause the insertion of additional blank pages which may make the "# Defined Pages" count inexact. This can only be checked when running a script, and at that point Q++Studio will issue a run-time warning. This type of occurrence should be a signal that there are ambiguities in the overall layout of your diary.