Based on your comment on HLGEM's answer, it seems you're trying to figure out when to break up into different career tracks. I'm assuming you're looking at something like "when do I break down the Engineer career track into Electrical Engineer, Software Engineer, Mechanical Engineer, etc."?
If this is really your question (or even if it's not), I'd answer "Break them up when it makes business sense." If the specific responsibilities of one job are different than another, but the expectations for promotion (i.e. performing work independently, leading others, reviewing designs, etc.) are similar enough you can leave them together. On the other hand, if you pay a software engineer differently than an electrical engineer then this would be one reason to not just have them both in an "engineer" career path. You should resist making separate tracks just enough to reduce unnecessary complexity. You also don't want to change these very frequently, so resist change to an extent so you can keep from creating, then removing titles later.
You should also consider that there are forks in many career paths. You want to enable people to pursue careers that fit their strengths. For instance a software engineer may decide to go down a technical path or a managerial path. You could have an intermediate step of "principal engineer" and "lead engineer" title to express the different focus of their roles even before they completely diverge, but isn't required. Eventually one would become a "technical architect" and the other a "development manager" or something like that.