I think of artlangs, auxlangs, and engelangs as the three different types of conlangs. There is overlap, of course. The main difference is the intention of the creator.
When I think of an artlang, it is a conlang for which the aesthetic components were the most important to the creator (Quenya or Na'vi). An auxlang is a conlang that the creator hopes will become useful as an IAL (Esperanto). Whereas eneglangs (Lojban or Ithkuil) are not primarily about the aesthetics, or worldwide propagation, but are engineered to achieve certain goals.
But this distinction is fuzzy. Aesthetics are important for auxlangs, too. And there is an "engineering" component in the design of the syntax of an artlang.
Mostly, though, people use the terms conlang/auxlang as you describe.

On Sun, Jun 8, 2014 at 2:32 PM, Paul Bartlett <[log in to unmask]> wrote:
On 6/8/2014, Jeffrey Brown wrote:

That is perhaps a reason to use "engelanging" for the creation of, but
"auxlanging" for the promotion of, an IAL.
For is not an auxlang a type of engineered language, one whose design
goals are the ease of use and learning and world wide adoption?

Yes, but I think that there is a useful distinction between the terms "conlanging" and "auxlanging," as well as engelanging."

By definition, an international auxiliary language is a language used between / among persons of dissimilar native languages who do not have or do not choose to use both / any of their native tongues in common (or, some instances, deliberately choose to use the language of one). In this sense, English is the most successful international auxiliary language to date in world history. It just is not a constructed language. Other examples, of more restricted scope (compared to English), have been French, Latin, Koine Greek, Persian, Aramaic, Assyrian, Sumerian, and (some) Chinese.

In general, "auxlanging" as the term is used today, according to my experience, refers most commonly to the development, promotion, and advocacy of constructed auxiliary languages. "Engelanging" is a relatively new term which (so far, at least) seems not to have caught on. (To me personally, it seems unnecessarily clumsy and unneeded.)

I myself, based on my experience of many years in the area, consider the term(s) "engelang[ing]" to be both superfluous and unnecessary. If you (figuratively speaking) are interested in constructing a language particularly for hobby, linguistic, theoretical, play, literary (in the manner of Tolkien), or other similar interest, you are a conlanger. If you want to develop or support a particular constructed language as an proposed international auxiliary language, you are an auxlanger, and you do not need to call yourself as an "engelanger." Obviously you can be both at the same time, and you can propose a language both as an auxlang and a conlang. But I think the interests are distinguishable.

Also, as has been obvious in this group within the last few days, people can discuss natlangs as auxlangs, as this is perfectly legitimate. In common usage "auxlang" more commonly refers to a constructed language, but it does not have to, but auxlanging is distinguishable from conlanging generally.

Paul Bartlett