Well, in the large commercial projects we're dealing with there are plans of course. I recently looked into a fairly new commercial panel and found some 40 pages of diagrams and plans . In residential construction things get more interesting... usually no documentation at all, especially in multi-family buildings the owner gets the documentation and any electrician called by the tenant gets to guess.
A couple of weeks ago my cousin asked me to look over the wiring of his newly rented early 1980s apartment (many tenants here prefer to fix up the place at their own cost rather than argument with the landlord to get things fixed even if the landlord were legally obliged to do so). Opening the panel I found everything nicely labeled. "Range", "Dishwasher" "Washing machine", "plugs and lights kitchen+bath" and "lights and plugs everything else". Talk about load distribution...
Devices had been replaced by previous tenants, so labels on the device wouldn't have helped. Junction box covers had been removed and the walls were just wall papered over the open bixes... so no help either.
It took me quite some time to ring out all the wiring and for example determine where the supply for the rotted outdoor wiring originated (I had to disconnect it until it gets fixed and it's just tapped off the regular wiring at the weirdest points).
So my point is: in residential onstrcution there is hardly anything you can do to account for DIY stupidity, might as well label nothing except at the panel.
My grandmother's place is another nice example why panel directories are often useless... well, nice to know there are breakers "Plugs I", "Plugs II" and "Plugs III", but no idea what they actually control... (well, in fact so far I didn't find anything they control, all the receptacles seem to be connected to the lighting circuits).
