@rainynight65 thanks for asking :)
So the term 'expats' comes from 'to expatriate'. It was originally used for representatives of colonial rule that were sent to the colonies.
The implication was that they would eventually return to the motherland or be 'expatriated' home. This meant they were temporary migrants and had loose ties to the countries they were sent to.
This term is now used mostly by MNCs when they send employees overseas to work in a foreign office. So expats can be from anywhere, technically. Singapore expats in UK.. Indian expats in Canada etc.
In SEA, because of our history as ex-colonies, Europeans or Caucasians in general are usually assumed to be expats by the locals.
This isn't always true (which is a whole other thing), but expats usually make little effort to be part of the local community or forge meaningful ties with local residents.
Of course there are exceptions, but this is just a general overview. :)