Bitcoin Forum

Say, X is merit source for Russian sub-forum (https://bitcointalk.org/index.php?board=10.0). Will X be able to use his/her sMerit to merit Y on Service Announcements (https://bitcointalk.org/index.php?board=84.0)?

Yes. Merit sources aren't restricted to any board.

As a merit source for a specific board, you can surely merit posts you find good outside of that board. But most times they spend more of their Smerits in the board assigned them.
For example if X gets filled with 40 smerits per month, and X is a merit source for the bitcoin discussion board, X may spend up to 30+ merits on the bitcoin discussion board.

In addition to what LoyceV said there are no specific type of merit sources here in the forum. Yes we might have merit sources for specific local forums but their sMerits to give are for everyone not only for that specific local board where they use to send sMerits too. The ones who have the biggest merit allocation are not even for the local board yet they still hang out on a lot of local boards just to reward sMerits to good quality posts.

If a user chooses to award most of their sMerits in one board (as many non-English speaking members do for obvious reasons), then that is their choice, but there is no such thing as a "merit source for the bitcoin discussion board", or any other board for that matter.

As far as I know, we can use sMerit to all boards. Agree with @LoyceV, there is no restriction on it. So, certainly, he/she can use sMerit outside of Russian sub-forum. Do you have a problem with that thing, buddy? And what is wrong on service announcements?

There is no official restrictions on which board(s) one merit source has to use all allocated sMerits as a source. Although there are something merit sources should consider:
- It partially depends on details in their merit sources applications (if they did applied themselves, not automatically was chosen by theymos at the kickoff of the merit system). If one applied as a source with a statement that he/she would like to be a source in Altcoin boards (Discussions, announcements, etc.); that source (when accepted) should try to keep distributing sMerits in those boards.
- Nevertheless, if in Altcoin boards, in some weeks, is hardly to find good posts, it is fair to distributed sMerits to other boards.

Just my two cents.

As stated, yes Merit Sources they can merit on any board at will, although if they postulated for a specific board then they should probably give that board priority. Nevertheless, Merit Sources are not confined to read and participate on a specific board, so it is natural to perform their actions on any board they roam around.

As an example, the Merit Sources on my local board have, historically awarded 59,53% (one Merit Source) and 21,27% (another) respectively of their merits to my local board. That's quite a difference between them, but aligned to their reading and posting habits.

OP's confusion probably comes from the fact that some applicants apply as a merit source for a certain board. Like what others said, MS aren't restricted but, imo, they should atleast spend most of their sMerits to their local board. It's the reason why they applied and probably the reason why they got approved. 

It's also my understanding.
I applied in Meta for merit source position because I felt that we don't have enough MS in our local board,  Croatian.
I even have topic there where local users can report their good quality posts for merits https://bitcointalk.org/index.php?topic=4265795.0
But,  if you check reported or suggested posts you will realize that a lot posts are from English part of the forum,  mostly scam warnings.
I don't have time to read all forum so in such way I can find and reward good quality posts with merits.
I also reward other good quality posts from non Croatian members.