It's probably worth pointing out a couple of things that seniority is not. It is not a complete description of your influence; a street fighter who is given little recognition by his mob, but runs a private army, is going to be more influential than a nominal leader of that same mob who owns little but his pride (this mob is likely to undergo violent restructuring that will end up with the streetfighter in charge in the near future, but that's a different story). It is also not a guarentee of respect; wielding power certainly helps to get respect, but no more ensures it than, for example, being a company VP in real life.
Seniority does not work the same way in every organisation. For this reason, organisations in Horizon are noted as treating it as "casual" or "vital". In those organisations that treat it as "casual", seniority is more a recognition that you are a good leader than a promise to keep you in that role. You do not directly control many resources, although you will certainly find it easier to requisition them; you can give out orders to those less important, but they are not compelled to obey. In those organisations that treat it as "vital", seniority grants you the taste of raw power. You have control over the organisations resources, you can pull rank, people have to jump to attention when you turn your gaze on them, and so on. Seniority is more expensive in such organisations. For example, at the Imperial University, buying seniority gives you all manner of pretentious titles, and the right to wear gowns that would look absurd on anyone less learned than yourself. But basically it's up to you to win funding for your projects, recruit assisstants who won't destroy the lab, and occasionally persuade the Watchdogs not to arrest you. Seniority is "casual". In the Watchdogs on the other hand, seniority gives you large groups of armed men as your (sort of) minions, the right to incarcerate pretty much anyone if you can think of a reason, and the right to raid people's property. Seniority is "vital".
In general, if you have bought the relevant quirk/profession to be in an organisation, you have seniority one. Mobs also have seniority zero, which indicates that you are regarded as that mob's "property" (or "client", to put it tactfully), but have no official status. Most probably you pay them protection money or some equivalent. Everyone gets rank zero in a mob at character generation based on the district they live in, if they don't buy full membership of a mob or get around the issue some other way (buying seniority in the University, or something else unrelated to the mobs, does not get you out of paying protection money). You can buy seniority one (and no higher) in an organisation you are not a member of at the cost indicated below; this means that you are sufficiently important to the organisation on a long term basis that you have some authority within their ranks. The GM team will also need you to decide why you are so important to the organisation.
Seniority, at least in the uniform way it's presented here, is an OOC concept. Some organisations don't even have an overt power structure; at the other extreme, groups like the Mayoral Legions have far more than four ranks, and these are supposed to define the members' entire professional life. However, it all gets dealt with under the same system. IC characters do not discuss (or even know) their seniority level as such. If your level changes during the game in a non-obvious way, we will let you know.
Seniority costs as follows: two points per level where it is "casual", three points where it is "vital". Remember that rank one in an organisation comes free with whatever quirk you used to join it. You will notice that if you want to buy seniority four in an organisation where it is "vital" it will cost you a whopping nine points plus the cost of joining in the first place. This is because you are more important than most of the city government.
Here's a rough guide to what seniority levels means. High rank comes with free stuff; where we say that purchases of a certain level get made for you, note that there are very firm limits on this. We're not going to put a system on it, but abuse the requisitions and a GM playing someone from accounts will approach you with gloomy looks and, depending on the organisation, possibly a very big stick. You were warned. This system applies to getting hold of things no questions asked; if you have a really good resaon, your organisation may spring for it whoever you are. Also, in organisations where seniority is 'vital', anyone at greater than one automatically gets a small group of lower ranking minions. This is in addition to any extra mooks you buy.
0 (mobs only): you are well above the level of property, but still not exactly a full human being. You happen to live in a mob's area of the city and haven't (yet) been able to get out of paying them insurance. Still, you are regarded as in some way "part" of the mob, and can expect some protection in return for your money.
1: You are a front line troop, a pen pusher, or other such subordinate. However, you are now one of the boys, a full member. If seniority is vital, you can expect purpose to be forced into your existence on a regular basis by your superiors. In a mob you may be someone's mook, or you may be a semi-independent operator who is considered to be of little importance. Alternatively, you may be a non-member who is really important to the organisation for some reason. Any way, you can beg the organisation for stuff and it's possible they'll listen.
2: You are trusted with a little authority. In a mob you might lead a squad of goons; in an army you're a corporal or sergeant. If seniority is "vital" you get a Poor income, if you haven't got anything better from another source, and can try to get trivial purchases bought for you.
3: Middle ranking. A captain in an army; someone who heads up permanent operations in a mob. Outsiders who know about your group on a casual basis may have heard of you just because of your position. If seniority is vital you get an automatic Moderate income, and can try to get Minor purchases made for you. If it is casual you get an automatic Poor income, and can try to get Trivial purchases made for you; also, you will have a few assisstants or equivalent paid for by the organisation.
4: Leader/crime lord. You are a professor at the university, a senior commander in an army, part of a governing council, or one of a mob commander's immediate henchmen or deputies. Outsiders who know about your group on a casual basis have probably heard of you just because of your position, unless you've taken efforts to cover your role up. If your organisation is at all respected or notorious, you will have a small amount of political clout, again just because of who you are. If seniority is "vital" you get an automatic Comfortable income and can try to get Notable purachses made for you. If seniority is "casual" you get an automatic Moderate income and can try to get Minor purachses made for you; you will also have a small group of underlings.
Not even seniority four makes you the head of one of the big organisations. If you want to play one of these powerful game world figures you can come and negotiate seperately with us. We will then make a public example, sorry special case, of you.