This is the accepted process by which various interested bodies establish the limits of their respective professions. On Roundworld, it was taken so seriously by trade unions that if one worker stepped across the line and performed work that was considered to rightly belong to somebody else, it could provoke strikes and unseemly fraternal snarling amongst comrades. For instance, if you called an electrician out, there was no way he could break into a wall to uncover buried wiring, ooh, no way, guv, you need a builder to knock through to the wiring, it's his job, I only do electrics. And ask him to re-plaster when he'd finished with the wiring? Ooh, more than my job's worth, guv, that's a plasterer's job, catch me taking his livelihood away? And once you had a plasterer in to cover up the mess left by the builder and electrician, would he repaint or repaper the wall? Stone me, squire, you need a decorator for that.... Flanders and Swann famously wrote a song about this. ("The Gasman Cometh" — relating the visits of a succession of tradesmen, each coming to repair the damage done by the previous tradesman).
On the Disc, the growth of the Guilds has prevented outright bloodshed, although it has been a pretty close-run thing. Witness the debate in The Colour of Magic between Stren Withel (President of the Thieves' Guild) and his counterpart Zlorf Flannelfoot (head of the Assassins' Guild) about the respective roles of their two professions. This was done in the bar of the Broken Drum with weapons not far from plain sight, and took the form:
"You rob them. WE kill them."
This sort of demarcation, between two Guilds with well-defined and time-honoured areas of professional responsibility, is fairly straightforward and easy to sort out as between respected professional equals. Indeed, any Thief who hits the mark that tiny bit too hard over the head is likely to end up as an extra windcock on a roof somewhere, or with both knees nailed to a central support of the Brass Bridge at low tide, by his own Guild, as assurance to the Assassins that they take such breaches of protocol seriously. Similarly, any Assassin who gives in to temptation and takes an unauthorised souvenir away from the scene of an inhumation is likely to be disciplined by the Dark Council, as a courtesy to the Thieves.
Witness the disciplinary action meted out to ex-thief Bundo Prung for an example of this in action.
The wizards aren't going to like it. They're going to call it magic. You know they get really pissed if they think you're doing magic and you're not a wizard. (Alchemist Lully to guild president Thomas Silverfish in Moving Pictures)
Where demarcation could cause trouble and strife is in the newer professions. Regard the emergence of soft-core porn magazines and strip clubs on the Disc. These trade on sex, long the sole preserve of the Seamstresses' Guild. Yet a new Guild, the Guild of Ecdysiasts, Nautchers, Cancanieres and Exponents of Exotic Dance, has arisen to defend the interests of strippers and pole-dancers. Could there be a dispute emerging here?
And how far is the Guild of Conjurors a separate entity from the clowns? Both trade on an art-form that has become fossilised and stilted beyond belief - as well as transcending normal entertainment values- with the passage of years, and in both cases not a million miles removed from mime artistry.
And how do the doctors feel concerning Doughnut Jimmy's occasional forays into medicating human beings?
Does the Musicians' Guild extend its writ to singers, as Agnes Nitt spent her first few weeks in Ankh-Morpork trying to make a living by singing in taverns - almost certainly as a non-Guild member? (As a latent witch, what might have happened had the Guild tried to enforce its sanctions on her...). Does the Opera House pay Guild surcharges on its musicians and singers? (Andre is a Guild member, so presumably yes?)
The final court of arbitration must be the Patrician. But which sensible Guild member would draw Vetinari's public attention down on the fine details of their practices? There is always a sting in the tail to his judgements...
Finally, even the Gods themselves respect demarcation lines. In Small Gods, Om informs Brutha that while lightning is the common prerogative of all Gods, there's Hell to pay if you add the thunder off your own bat - you have to sub-contract for that and get Blind Io in to do it. you get the thunder, he gets a share of any belief engendered, everyone's happy.
And in the computer game Discworld Noir, another issue of demarcation is raised among Gods. As Cephut, God of Cutlery, points out in a heated discussion with War Gods, there isn't any clear-cut dividing line where your domestic cutlery stops being something peaceable you set out on the supper table, right, and starts being a weapon of war. I mean, this fork, right, to a gnome, it's a trident? And this Tsortese Falchion, right, what's to stop it being just a large knife, and therefore my department? Large knife, small sword, no clear cut dividing line...