I received a clearance to "join left downwind" from ATC yesterday
For the benefit of those newer pilots reading this thread, it's probably worth clarifying that that wording sounds more like an instruction, than a clearance. Without the context of the communication, it's hard to tell, bu if the phrase "cleared to..." was not included, the "join left downwind" part was not being offered as optional. A clearance will usually include a description of a limit, meaning that you are cleared to fly as far as XX, with the expectation that before you get there, you'll have a further clearance (so you don't have to stop and hover).
While flying many flights in a more complex terminal area last week, the difference was clear: Sometimes I was "cleared to descend to xx", meaning I was not to fly lower than that altitude, but also not required to get down there either. Other times, I was instructed to descend; "C-GABC, descend to xx", meaning I was expected to get lower, so someone else could fly over me. It's worth understanding the difference between an instructions and a clearance, failing to comply with the instruction is likely an offense. Failing to comply with a clearance would not be, though exceeding it would: "G-AXYZ, cleared straight in final" is a clearance, to approach the airport on the runway heading, descending, but if you were to land without further clearance, you'll be in trouble. Similarly, if you hear the instruction "G-AXYZ land...", you
are in trouble!
When I first started flying helicopters, I would fly my solo cross countries to airports I knew very well as a fixed wing pilot. Of course, the controllers did not hear a a fixed wing pilot on the radio, they heard a helicopter pilot. So I would get all kinds of clearances I was not used to: "C-FMMR, fly direct to the main apron". That was actually an instruction, and for a fixed wing guy, about to cross the departure path of the active runway he knows very well, it's a little disconcerting. But, I realized that I would completely confound the entire place, of I asked to fly a fixed wing circuit the to runway!