In addition to specific specialisations, you can choose whether you want to play as a Freelancer or a Fellowship or Guild member. Just as your career is constantly evolving throughout the game, so might your career plans as well. Thus, if you ever decide that being a lone wolf isn't fun anymore, you can apply to join a fellowship or a guild or even choose to start your own. Similarly, if you're fed up with being a fellowship or guild member, you can go back to basics and become a solo thief again. All three possible career paths have their advantages and disadvantages, obviously, but the even more important thing to remember is that neither a freelancer, nor a guild, could survive without cooperating with each other from time to time. As with many things in the thieving world, it's often a matter of balance and interconnectivity…
Freelancer - For the lone wolf in every one of us, it might be desirable for some people to focus on working as independents. On the plus side, no hierarchy and no responsibilities to others will be dragging you down in your professional thieving career. On the downside, you're predictably all alone for everything and have to fend for yourself and your daily needs without any hope of external backup. There's quite a few freelancers operating in Melza and Aporue, but being one is not as inexpensive or easily manageable as it might look at face value, so the absolute numbers of such thieves are actually quite small. Long-term veterans are quite few and far between, since most freelancers either retire early, join a group of thieves, or… well, just don't make it.
Fellowship - Fellowships are something of a middle ground and halfway house between being a complete freelancer and being a member of a guild. They can most easily be described as “clubs of professional thieves, smaller than established guilds, and lacking the more official structure of guilds”. Fellowships are thus much less formal in their power structure and hierarchy(-ies) than a real guild, and this goes hand in hand with the number of their members usually being in the single digits. Even the biggest fellowships never exceed more than 15 or so members. Fellowships are inherently small organisation. And while it might not seem like that, fellowships can be quite numerous, generally much more than guilds (due to lower demands for infrastructure) and quite a bit more than freelancers (as freelancers have it tough, due to not being able to rely on the advantage of numbers). If you're unsure of running a guild from the get-go, or you simply prefer the greater simplicity and casualness of a looser club, a fellowship might be the right choice for you. As you'd expect, there are both advantages and disadvantages to a fellowship. The plus side is that it has actual membership (impossible with a freelancer) and this allows it to be more sustainable in times of need, when mutual assistance among thieves can prove absolutely crucial. In addition, the informal and more relaxed nature of a fellowship makes it somewhat easier to govern and its members often tend to have a greater sense of “being like a family”. However, the often overlooked but ever-present downside to a fellowship is that exact same low number of members, which can make a fellowship vulnerable to a better organised rival, especially if it's a gang or other criminal organisation from outside the more tolerant thieving underworld. And last but not least, the much less formal structure can make fellowships die prematurely due to infighting and other internal power struggles, as well as more vulnerable to the aforementioned external efforts at breaking it up.
Guild - Guilds are the largest, most institutional and economically and power-wise most stable organisations of professional thieves in Aporue, Melza included. There is some overlap between a fellowship and a guild, but to be perfectly frank, it's purely a matter of developmental stages of such organisations. Some successful or expansion-minded fellowships tend to increase their membership and influence over time, to the point where they are simply too big to function in the established ways of a fellowship. At that stage, sooner or later, one has to cave in, do some changes, maybe establish an in-guild charter, secretly buy new safehouses for the increasing membership, “move upmarket” in a city's thieving underworld, etc. Times change, conditions change, decisions need to be made. If you make it all the way to establishing a guild, congratulations ! But it's not so straightforward a thing: While on the plus side, establishing a guild means an extra haven of stability to professional thieves, the obvious downsides are that such an organisation needs to be fairly long-lived already at its start, and as a larger body is harder to manage, more difficult to keep alive, and is even more prone to attract the unwaned attention of rivals in the thieving business - or worse, attract the attention of other organisations in the criminal underworld, one's often seeking to blunt a guild's influence and successes. Pick your poison. Running a guild is great, it can be a real passion, but it always comes with more than one catch and one has to learn how to fend for his organisation and fellow members.