I have not had the time so far to look at the layout-manager, and i was not aware of the "xowiki includelet" so far. The term "includelet" is used in xowiki since a few years and refers to on-demand-generated chunks of HTML provided in an objects oriented manner (can be subclassed, etc.). These includelets have certain common properties like decorations, titles, and support different caching strategies.
The term "includelet" has been used for years for bits of includable template within the toolkit. IIRC Lars Pind was the first to start using it.
See, for instance, "acs-lang/www/change-locale-include.tcl", written by one of Lars's ex-employees, Peter Marklund.
The layout manager is using it in the same sense as the old use, essentially a declaration can be made that identifies such an includelet so that the package can manage it.
Here's another example from the assessment doc:
An editor wants to conduct a poll on the site with
immediate publication of the result to get a feeling how users like the
new design of the website. The result can be displayed in an includelet
(see the below for details) on any page the editor wants.
They're not talking about xowiki.
I know the terms been used informally in discussions among team member for years, entirely outside the context of xowiki.
