Immigration District: what is it ?

[YetAnother]

heard recently a credible reference to 'Immigration District'; up until then i was under the impression we could only use the imm office we were/are 'registered in' (i presume that means the most recent address they have for us (example change of address form));

more recently however, i am hearing that residents of Chiang Rai city have successfully used the Mae Sai office and there are strong insinuations that we, here in Chiang Saen city, can use the Chiang Khong imm office (was told that the Chiang Khong office is the 'mother' of ours here in Chiang Saen);

might someone more knowledgeable than i weigh in on this ?

TIA 

Edited November 30, 2017 by YetAnother

[ubonjoe]

The office in Chiang Rai city is a sub office of the one in Mai Sai which means you can use either office. As far as I know those are the only offices that can be used for extensions and etc.  The others are border checkpoints.

The designated areas is normally done by province.