Hobby Skill is a category of skills: anything that is studied as a hobby, and isn't already defined as a skill. Hobby Skills may be DX/E or IQ/E, and normally default to DX-4 or IQ-4.
Professional Skill is another category, created to cover the specialised skills of non-adventuring jobs. Professional Skills are DX/A or IQ/A, and usually default to DX-5 or IQ-5, if they have defaults. Some Professional Skills have prerequisites of standard skills needed to do the job, such as Electronics Operation (Sensors) for an air-traffic controller, or provide additional defaults for other skills, such as Journalism providing a default for Typing.
Hobby Skill and Professional Skill are also
placeholders for "New skills that are Easy or Average", and you shouldn't read much into the names "Hobby" and "Professional". They were introduced at 4e, as part of cleaning up skill bloat.
Several 3e skills became Hobby Skills at 4e, including (Juggling), (Kite Flying) and (Origami).
Action adds (Flower-Arranging).
Boardroom and Curia points out that hobbyists might use Connoisseur rather than a Hobby skill. Basic has (Martian Orchids) on an iconic character.
DF5 adds (Firestarting), which since it's on a salamander, is probably for arson;
Dragons has (Needlework),
Madness Dossier (Whistling) which might well be HT-based, and
Locations: Metro of Madness has (Comic Books), (Metro) and (Pop Music).
Martial Arts has (Feats of Strength) and
FCCS has (Secret Inks). Mysteries has (Trivia).
PU3 and
PU7 treat all Hobby Skills as falling under the same Talent or Wildcard Skill, given their limited usefulness.
Underground Adventures has (Rock Collecting), as a cut-down and easier version of Geology; many other scientific skills can be hobbies in this manner.
3e skills that became 4e Professional Skills include (Bartender), (Cooper), (Distiller), (Dyer), (Florist), (Glassblower), (Prostitute), (Tanner) and (Weaver).
Action adds (Firefighter), which has a good mechanic: it can be used as a complementary skill to any other skill for firefighting tasks. Basic adds (Journalist),
Crusades (Netmaker),
DF7 (Mortician),
DF12 (Brewer) and (Server),
DF15 (Torturer), and
Fantasy (Courtesan) and (Midwife).
High-Tech supplies (Distiller) and (Typesetter).
Horror adds (Nurse), and
Hot Spots: Constantinople (Master of Ceremonies).
Locations: Metro of Madness has (Banker) and
LTC1 (Scribe).
LTC3 has a
lot of the professional skills needed to run an economy, many of which continue at higher TLs. We could use an HTC series to fill out details for TL5-8, such as (Bricklayer), (Plumber), (Gas-worker), (Glazier), (Painter), (Plasterer), etc., but I doubt it would sell well, and the Yellow Pages is a decent substitute.
Magic has (Barber),
Martial Arts (Modelling) and
Gladiators (Stagehand).
Mysteries has a more detailed version of (Firefighter) and adds (Law Enforcement). Unlike Hobby Skills, Professional Skills are counted individually for Talents, and it is rare for several to fall under the same adventure-orientated Talent.
SEALs in Vietnam has (Plumber) and (Welder),
Supers has (Curator), and
Chinese Elemental Powers mentions (Healer).
Thaumatology: Urban Magics has (Urban Planner) and (Contractor).
THS: Changing Times has (Aquaculture) as an option,
Ultra-Tech has (Massage), and
Zombies positively emphasises (Mortician).
Soldier is an example of a well-developed Professional skill. It includes knowledge of basic tasks that would fall under quite a few other skills. For example, using a radio for routine tasks without learning Electronics Operation (Communications). Are all professional skills like that, or only some of them? If some, is there any way to decide other than GM's Opinion? There's a
way to decide what capabilities fall under skills that are like this, but not which skills can do it.
I've created a couple of Professional Skills for a WWII campaign: (Factory (Munitions) worker), where a professional skill for working around explosives seems justified in addition to Machinist or another general factory skill, and (Visual Signaller), for naval signalling with lamps, flags and semaphore: most naval ratings would use Seamanship for this, but there were definitely specialists with much higher skill, who'd often make Per-based rolls, and weren't superior at everything covered by Seamanship.
What character hobbies or professions have appeared in your games?