SFW vs NSFW

Does "SFW" just mean "not NSFW", or does it mean "actively safe for work"? I guess, they are broad categories, but I'm not sure when to interpret SFW as "typical of the SFW category, eg. societally acceptable prose" and when to interpret it as "right up against the boundary of NSFW or I wouldn't have bothered to specify, but not over it" :)