Guilds
The system of guilds is spread across Arthea; the quality of guild support for given professions varies from industry to industry, but in general the richest industries rely little on guilds, and so have developed few professional bodies, while the poorest have the worst infrastructure and can barely afford the overhead of such organisations, rely far more upon their collective voices, despite the low levels of contribution they receive. For these groups, the guild is the only protection from corporate dominance and political interference. The best guild networks span the cities of the nations, while the poorest may meet in just a single guild building, and may find themselves competing with similar guilds in their town or city.
Professions that rely on their guilds require many common aspects from the network; training, supply networks, profession-based advice and support, protection from legal challenges, and a unified political voice are all common. These apply equally to tradesmen's guilds and the higher (and lower) status guilds that appeal to player characters. At times it seems that the local guilds are all that stand between artisans, craftsmen and traders, and injustice and dominance of a misguided law.
There are many trades guilds supporting all kinds of professions across Arthea, from abattoirs to zoologists, but the ones of interest here are those that directly affect player character classes and the non-class services they need. Clerics and monks have their churches to rely on, wizards and sorcerers have their Sects, which are an order of magnitude higher in terms of organisation and training (but often not in size), the others are are supported by one or more of the guild types that follow.