Professional Skill is the mastery of all the metrics and rules of thumb, and processes of a given profession. Metrics are the measurable or observable values that matter in the given profession. Rules of thumb are intuitive and abstract values and what to do or how to react when encountered; they are what to do with a given metric.
An example of rules of thumb are the Ratios. In every profession there are metrics that have to keep with certain ratios that help identify assumptions and how to identify root cause of problems. A very well known example of ratios is the Bakers Dozen, a baker knows he always needs to make a little extra in case of accidents, presentation purposes or sampling.
If the profession is part of a larger organization there comes much larger and greater amounts of ratios to track and understand. In a Low-Tech setting bureaucracies, soldiering, guilds, and organized religion have very complex set of ratios.
Example of Ratios for a soldier:Organizational Ratios, how many soldiers to one lesser leader, how many smaller teams to a leader, how many men to a company, how many companies to higher leader etc. Then there are how many support staff for an leader; how much supplies per soldier or team or section or squad or company or army etc. per day, per campaign season, per year. Then there are the ratios of their opponents and their own organizational ratios.
Guilds, Bureaucracies, and organized Religions have similar ratios to be mastered. Social Elites also have such ratios to master, but you can base them mostly on bureaucratic profession. This is because some social classes serve as the Literate Elites: examples are Senatorial Class, at one point the Equestrians, Various Priestly Caste of various cultures and the Mandarins.
Processes are also a big part of Professional Skill. Processes are the way things are organized and done; processes also affect etiquette and internal status of an organization and profession. These sometimes are so extensive in Bureaucracies and Merchantile Organization, like Guilds or Merchant Princes; that run States or Major Economic Polities that they fall its best captured in Administration Skill. Professional Skills can default more complex feats at -5.
Chain of Command and Escalation. Who you should report too, who is their boss, and who is your boss' boss' boss. This also includes how to address them, show respect, present one's self or present information.
Vetting and Checks and Balances. Processes are also more important in making sure an action is executed well. This is a more elaborate form of Chain of Command and Escallation, with the purpose of getting things done right and maximizing the mental resources of an organization. Of course some organizations are worth crap, and it shows in their Vetting and Checks and Balances.
These are the two primary purposes of Internal Processes in the Professional Skill, anything more technical deals with other skills outside the Professional skill.