Well, if it's in the wrong place and causing a problem, it will just get moved by one of the mods usually. I wouldn't worry about it too much.
Here is what I have gathered as a user, so of course it may be inaccurate or variable. Basically, if one of the main themes is suicide, it goes in SS, otherwise if it's more general discussion of a topic or suicide is only sort of relevant to it, it probably goes in Offtopic. Of course there is overlap because most people here are suicidal and are gonna mention that.
- Discussion of methods, what makes you suicidal, your own suicide or suicides in the media, suicide as a topic (so suicide law, suicide stigma, emotions related to suicide, etc.), farewell messages, announcements of an attempt at recovery, anything else primarily pertaining to suicide all go in Sanctioned Suicide, a forum for discussing suicide
--- Speculation on other users' suicides belongs in the megathread in Offtopic, though.
- Borderline stuff: Afterlife discussion, personal circumstances (like illness), effects of depression and other mental illnesses, discussion of the negative aspects of society (e.g. isolation), discussion of non-suicidal people, etc., discussion of the community
--- Users choose where to put their thread and it gets moved if it's inappropriate. The main thing seems to be, if it's related to active interest in suicide, it goes in Sanctioned Suicide, if it's more general or not necessarily suicidal, it can go in Offtopic.
---- So threads ranting about society could go in SS tagged as 'Venting' if you want support or to discuss it in relation to suicidal ideation, whereas if you just want to speculate about society itself, it's more likely to be in Offtopic. There is a general afterlife megathread in Offtopic because people here like to talk about what the afterlife could be and even debate it, but people sometimes like to talk about their specific fears of the afterlife in SS and how it affects their thoughts and feelings towards suicide.
--- Discussion of the community goes in Feedback if it's aimed at the mods or a response is sought for them. If it's just directed at the users, it seems to be more acceptable in Offtopic.
- Everything else goes in Offtopic or Feedback