thanks

that's mostly a legal term. it's our heritage from the Soviet Union. back then everyone had (or at least was supposed to have) "прописка" (not sure if the term is translatable, I guess in English it's also propiska). so everyone had (and still has) a stamp in their passport, where their permanent residence address is written. a lot of things are connected to this address. for example
up until just a few years ago you were able to get free medical treatment only in the hospital, that services your propiska address. it is still used as identification. in many official documents you may find something like "Гражданин Петров Сидор Модестович, 1964 года рождения, проживающий по адресу... "
Back in the USSR mobility was not approved of, so this was very useful for tracking the whereabouts of people.

Igor thanks for this informative comment. .l had no idea about how soviet system was working

Nowadays, of course, we're not that totalitarian (at least for now) so it's not that severe. you can move around without changing your propiska, but if you stay somewhere for more than half a year or so, you have to register there. it's called “регистрация по месту временного пребывания” or some such. usually it's called just регистрация. Moscow police for example is infamous for checking weather people have this registration and taking bribes if they don't. of course tourists don't have to have it, but illegal workers without registration are rumoured to be the main source of income for Moscow police.
but I got swept away a bit, but the whole point is that we have two types of citizens' registration - permanent (прописка, also called постоянное место жительства, or just ПМЖ), and temporary (usually called just регистрация). so постоянное место жительства is the address, written in a person's passport, where they're supposed to (but don't actually have to) live.

you explained it perfectly. ..by the way Your English is very fluent as l see

and one more thing, there was a phrase back in the 80’s and 90's : “уехать на ПМЖ (постоянное место жительства) в... ” - wich means to leave the country for good, to emigrate.

you're welcome, David. I'm always glad to share what I know about Russia, it's history and language. and thank you so much for complementing my English. I really appreciate that =)