I think this is a great question. Something I've wondered about before too, in a theoretical camfire discussion kind of way.....
I find a lot of this discussion to be a little ...well pendantic
I'd be very curious for a controller or three to chime in with their comments...and from different perspectives...en route vs terminal, etc...
Do they enter such things into the computer and it plots a course line? Does that line get passed on to controllers in the following sectors?
Just thinking about it logically
I think that it's clear what folks have said here that direct means direct. In theory, when you're cleared direct from point A to point B that this line, or really path of some width has been determined to be clear for you...traffic conflicts, terrain, obstacles, airspace issues, etc...
So my assumption is that yes, in the short distances say for example in the terminal area, that's probably a fairly narrow path you're cleared for and it's expected..... of course...absolutely.
but do they have enough memory bandwidth to really keep track if you've drifted off course a couple hundred yards? maybe 1/4 mile off ok maybe.... 5 miles off, alarm bells go off, or whatever...They're continuing overwatch anyway, and even if you stay on course if conditions change they'll amend.
Regardless, clearly that line is the intended clearance and that's the path they want you on.
Now take that same scenario put in the context of en route. Say I'm in Florida and I depart for a destination say in Kentucky.... and I'm cleared direct to some IAP up there. Of course it's expected to follow that line....but realistically they probably aren't going to remember with precision, I'd think they are just expecting you in that general direction... in this case once I'm way up into North GA and I'm off course 6 miles, does anyone really even know?
not good...true enough
theory says I should correct and get back onto the intended original course ASAP. Yes, Absolutely
But real world?