I received this (really good) question a while back, and neglected to answer it at the time. Fortunately the author gave me another shot and asked again the other day. Special thanks to Steve from bringing this one forward.
Here is the question:
I was wondering if at the international level ( and others too if different ) if referees are designated as either center or assistant ( or 4th )? Really trying to ask if the same referee will always be in the center or as an assistant or if individuals will take assignments at both position. For example I have seen countless matches with Howard Webb in the middle, but does he ever work the lines?
My answer ranges from "yes" as a matter of procedure at one end of the spectrum to "not always" in the middle to "yes" as a matter of practically at the other end of the spectrum. Let me explain.
US Soccer currently has (12) active grades in the US and several emeritus grades for referees who retire and keep their grade as an honorary title. A copy of the list can be found here, but the most current and complete list can be found in the Administrative Guide for referees at US Soccer.
If you look at that list there is no place where a position is noted except in Grades 2 and 1, International Assistant Referee, and International Referee respectively.
So plainly speaking, if you are wearing a FIFA badge, you must be performing that function commensurate to the badge you are wearing at the international level. If you are wearing a FIFA referee badge, you must be refereeing, or acting as alternate (4th) official. If you are wearing a FIFA AR badge, you must be running a line.
This does NOT mean that all a FIFA referee or AR will do forever is act as a referee or AR. Take a look at the last line on page (2) of the guide. It states:
*International Assistant Referees must wear the USSF National Referee badge when assigned as
referee in all matches.
FIFA AR's when they serve as referee at levels other than International, need to wear their National badge. Take note that FIFA referees need not do so when serving as an AR in other than International matches. It is my experience that these folks do "other" matches and in different positions from time to time.
So at one end of the spectrum, the international level, Referees and 4th are always that, and AR's are always that as well. At this point, at the MLS level each referee is on a path of either serving as a referee, or as AR in nearly all cases and at that point, it will rarely change.
Take great note that sometimes this path is not always the choice the individual would make, and at times bases itself in other more unconventional reasons beyond raw talent, ability and performance. These reasons could include age, space on a particular international list, geography, and yes, political savvy of a candidate, or their state association.
This was not always the case. It was only until the mid-90's that FIFA made this delineation between referee and AR, and one quite frankly I agree whole heatedly with. Being an AR is a specialty, and a really tough one. I personally was very lucky as when I was coming up the ranks and got to serve as 4th, AR, and referee in the MLS, eventually being asked to specialize as referee.
When you are asked to specialize that is all that you wind up doing generally. For example in my post Role Reversal, I opine about just how terrifying it was to pick up a flag of nearly a decade not doing so previously. Keep in mind though in the middle of the spectrum are amateur matches, and all referees will be asked to perform all duties at any time, so one will have solid experience to be able to perform in these other roles.
At the youth level, it is generally a mixed bag also, with an exception of just starting out. At that time I find that you will generally run lines to "get your feet wet" at local youth matches. This isn't a horrible thing as is gives a referee an opportunity to be a part of the game, without having the responsibility of match management yet.
There are several nuances to this, and a few funny stories too as this system was getting off the ground in the US, but hopefully this paints the general picture that one has to be competent in both disciplines through their career, but at the highest levels domestically and internationally, there is specialization.