First conference: choosing which session to apply to talk at Context: I'm an early PhD student (physics) applying for a talk at a conference (APS).  I have never been to a conference before. My topic does not fit neatly into any of the session titles. There are a few focus sessions and a few standard sessions that are more specific, and only borderline relevant (if at all, some do not have detailed descriptions, so I could be misjudging), and one standard session which is very broad and definitely includes my topic in its scope. For my abstract submission, I need to choose one of the sessions to submit to.

My goals:


Have my abstract selected. I want to have the practice of giving a talk at a large conference. 
Get feedback/questions on my work. Ideally this would be from someone who knows about my topic, is familiar with the literature etc. However, I would also welcome feedback on my general theoretical framework, which is general enough that a large fraction of theorists in the field will be able to understand after a decent introduction to the particular system that my work looks at.


Questions:


How do I pick a session which maximizes the chance of achieving Goal 2. while minimizing the damage to Goal 1.? I.e. if my talk falls outside of the scope of the session I applied for (but not the conference), will it increase my chance of being rejected? Or will my submission just be altered by conference organizers and moved to a different session?
Am I more likely to be accepted at a standard session than focus session?
Does APS allow for indication of interest in more than one session?
The APS March meeting is indeed a large conference, with around 50-60 sessions running concurrently. This scale means that they find a time slot (10 minutes + 2 minutes for questions) for most (close to all, it seems) submitted abstracts. If your work is at least of reasonable quality, you'll likely get that practice in. (Even if it is of dubious quality there is a chance. Google APS March meeting crackpot session if you want some entertainment.)

This scale also has the downside that it might be hard to get a lot of quality feedback on your talk - the right people might be elsewhere, and two minutes for audience feedback is not a lot. If you have specific questions, it's worth identifying experts ahead of time, and either try to schedule a meeting with them or just strike up a conversation in the corridors.


How do I pick a session which maximizes the chance of achieving Goal 2. while minimizing the damage to Goal 1.? I.e. if my talk falls outside of the scope of the session I applied for (but not the conference), will it increase my chance of being rejected? Or will my submission just be altered by conference organizers and moved to a different session?


You should just pick the category that's the best and most specific fit. Ask your advisor or colleagues for advice on which one that might be. Sometimes the descriptions are a little vague, and you can get a better idea by looking up the organizers' research interests.

If there are several reasonable fits, I'd tend to pick a focus session over a standard one, because it has a higher possibility of a) attracting people working on similar things, and b) having other talks I'd be interested in. However, keep in mind that a) is far from guaranteed, and there is a high variance that's out of your control. For example, if there is a relevant invited session at the same time, it will likely be more popular. (Protip: these tend to be the best sessions, along with ones specific to your interests.)

I have no numbers, but I doubt that there is a big effect on your acceptance chances overall for not guessing what the ideal session is. Also note that the list you see is one of sorting categories, not a final list of sessions. The organizers certainly can move your talk to another session - maybe even one not announced yet - as well as split and combine sessions. Maybe have a look at last year's program to see what it tends to be like.


Am I more likely to be accepted at a standard session than focus session?


To be clear, I don't think it matters for getting accepted to the conference in the first place. Whether your talk is scheduled in a focus or standard session is going to depend on your topic.


Does APS allow for indication of interest in more than one session?


I don't quite remember, but I don't think so. In any case, you'll find out yourself when you submit your abstract. (Maybe I'll update the answer next week after submitting mine.)