What are external migrants?

If you are asking for the meaning of the word "external" it means outside or 'out of' depending on the use in a sentence.. for instance an external light is one that you would have outdoors. An...

When compiling the source-program A, any symbol which is not defined in A itself (but somewhere else is defined) is called external.

Do you mean who can migrate? If so, in the EU anyone can migrate to one country to another, as long as both countries are in the EU. Elsewhere, people who are the subject of or have fear of...

People who move permentantly from one country to another.

Nope.