There is a difference between immigration and expatriatism. I think. It's semantics, of course, but from what I have observed, whatever difference is imagined is based on class, race, and nationality.
The term 'expat' seems only to refer to the affluent, particularly (though not always) those with Caucasian ancestry. The expat has no obligation to learn the language and customs of the place they live; the language of 'assimilation' does not follow them around their daily lives. And expats always have a home they can return to where they can enjoy safety, security, and economic opportunity. If they can handle the tax regimes.
'Immigrant' on the other hand is understood to mean a person who is motivated by a lack of opportunity in their homeland, or an ousting due to war or famine or corruption. The mythology around the immigrant is that they start from scratch with about five dollars in their pocket, and make what they can of their adopted home. Some 'succeed' by adequately assimilating and doing well in the private property department; others 'fail' to adapt and live out their lives in some sad littoral space.
The terminology is accorded based on colonialist ideas about which kinds of people mean what.
I've recently accepted that in my taking a job in publishing in South East Asia, and moving into a house with other English-speaking fugitives, that I am an Australian expatriate. I'm not ashamed — not really. It's just a fact of my life. I don't know how long I intend to stay, and I have opportunities back home that I can return to if things don't work out here. I am the kind of person who gets to be thought of as an expat. That feels weird.
Not because there is anything inherently wrong with being an expat, but because my morality is so sanctimonious, so staunch, that it is always in conflict with the reality of my decision. Many people are comfortable in the expatriate lifestyle; they make the most of their host city, and they cover their footsteps when they leave. Others are basically evil incarnate: people whose private staff in their way-too-big houses force me to wonder how they made it to adulthood without the basic skills of self-sufficiency.
Being a newly minted expat in Vietnam is rife with moral dilemmas: to what extent do I accept other people's choices to employ full-time domestic labourers? And without local knowledge,