This page explains the conventions and rationale for file naming. For information on the $path$ attribute, see $path$. For information about the significance of app.htm, for example in files named "Build.app.htm" or "out.app.htm", refer to File format.
The suggested naming convention for triple script source code modules is to name the file so that it ends with .src.
Early forms of triplescripts.org projects (including trplkt versions below 0.9.9) used file names ending in .js, but this was later changed because This is not JS.
The compiler itself doesn't care how the file name ends; it has no special reliance, nor does it provide any special affordance when a module's file name ends with .src. Nonetheless, it is a convention not to use the .js file extension for source modules as a strong signal to to reaffirm the triplescripts.org This is not JS mantra.
On occasion you might see references to files named e.g. Foo.xxx. The .xxx endings in this case is merely used as a stand-in for any suffix. During the time between the decision to stop using .js and settling on an appropriate replacement, documentation written that needed to refer to a given file tended to use .xxx as a workaround. To reiterate: from the compiler's perspective there are no special semantics associated with the ending of any file name, so neither src nor xxx mean anything in particular.