It's wasteful, and generally harmful, to have the same code repeated in multiple places, even if that code is HTML. A single definition means that only that definition need ever be changed, and you can't wind up with two definitions that are subtly different. There are two ways of avoiding this: fragment views and macros.
Fragment Views
Fragment views are the preferred method. When working with content classes, you may want to render a content class in a view for another class. The HTML you would want to include is usually the default view's HTML, stripped of its leading and trailing HTML, like <html>, <head> and body.
So given a default view named "+index", you would create a new view called "+fragment", and move the body of the HTML into it. The +index TAL would then include "context/@@+fragment" to copy the content in.
You could then reuse "+fragment" in other locations where you want to display that content object.
(The +fragment view can also be viewed in a browser by hand-hacking its URL. Since it's only meant to be included in other pages, allowing a browser to view it is not really pure. Providing it this way permits AJAX to reuse it, and normal users would need to hand-hack URLs to even be aware of it.)
Macros
In cases where you're not dealing with a particular content object, it may make more sense to use a macro. For example, if you're writing a page that seems to have certain things repeated frequently, you can define a macro for the repeated code, then use the macro later in the page.
Single-view Macros
To refer to the macro "mymacro" within the page where you defined it, you can use "template/macros/mymacro". ("template" refers to the template that is currently being rendered).
Context-wide Macros
If you have macros that need to be reused across pages for a given content object, then you may wish to create a view that holds the relevant macros for that content object, like +macros. You can then refer to it as context/@@/+macros/mymacro. It is rarely a good idea for one view to refer to the macro of a "normal" view, because that reuse will not be obvious to the casual observer.
Multi-context Macros
If you wish to reuse macros for views across various context objects, you'll need to create a reference to the correct Zope Page Template. You can do this by making a member of the view class which is an instance of ViewPageTemplateFile.
The view class would look like:
class FooView(LaunchpadView):
bar_macros = ViewPageTemplateFile("../templates/bar-macros.pt")A reference to the macro mymacro would look like: view/bar_macros/macros/mymacro.