A concept in air traffic control.
A 'release' is a set of conditions under which control of an aircraft is transferred from one sector controller to another. Airspace is made up of adjacent sectors of various classifications, which neighbour each other horizontally and vertically. Frequently, all air traffic in a particular sector is controlled by an individual controller (though it's not unknown for sectors to be combined during light traffic periods - this is called 'bandboxing').
An aircraft flying from one point to another may pass through several sectors of airspace. During its flight, the controller for each sector it passes through must coordinate, with the controller of the next sector, how they will 'present' that aircraft to them: where it will be and when, what altitude it will be at, and any other pertinent details. Coordination may take one of several forms, depending on the situation - voice coordination (either by phone or by controllers at adjacent positions doing face-to-face coordination), standing agreements (where a sector receives many flights going to a particular destination, for example) or the more recent computerised form.
Convention dictates that a coordination is performed at least ten minutes before the aircraft in question is expected to enter the sector in question, to give its controller adequate preparation time for its arrival. Coordination is only part of the procedure, though. There are also rules that dictate when the receiving controller actually has control of the aircraft, and what they can do with it when they get it; it is not uncommon for an aircraft to be instructed to contact the receiving controller twenty miles or more before it has actually reached their sector.
These rules - releases - may be codified in standing agreements or they may be agreed on an ad hoc basis between the two controllers. Take an example bit of airspace, viewed in profile:
+----------------+----------------+
| | STACKER |
| BLOCK | FL150-FL245 |
| +----------------+
| FL50-FL245 | DORP |
| | SFC-FL150 |
+----------------+ |
| |
-----------------+--------------x-+
///////////////////////////////////
///////////// SURFACE /////////////
Um, not to scale.
Now, x is an airfield, Nodehaven (sorry). Let's say United 748 (UAL748) takes off from there, and has a route that will take it out of DORP sector, through BLOCK sector on its way to its destination, say LAX. It has a requested cruising level of flight level 220 (FL220 - about 22,000ft).
Let us further say that aircraft regularly depart Nodehaven and go through BLOCK sector. So many, in fact, that DORP and BLOCK sectors have agreed that departures from Nodehaven via BLOCK sector will be 'presented' by DORP sector, to BLOCK sector, at FL100. This saves the controllers concerned from having to make a phone call every time a departure from Nodehaven goes to BLOCK sector: there is an agreement in place that negates the need for individual coordination.
Obviously this won't cover all eventualities (what if, for example, a flight from Nodehaven via BLOCK sector only wants to cruise at FL80?), but standing agreements are written to cover the most common categories of flight, since they are intended to save time.
This is all fine and dandy, except our UAL748 isn't too happy. He is climbed to FL100 by DORP sector and instructed to contact BLOCK sector. The thing is, he wants to climb to FL220, and at the time he told to contact BLOCK, still has 30 miles left before he reaches the boundary between the sectors, and the standing agreement states that the Transfer Of Control takes place at the sector boundary. So this poor guy is stuck at FL100 until the BLOCK sector has control of the flight.
Except that he isn't. The BLOCK sector controller, being the magnanimous, attentive and accomodating lass that she is, has seen from UAL748's flight progress strip that his requested cruising level is FL220. She won't have control of him for another 30 miles, but from what she can see of DORP sector on her screen, there are no aircraft that would restrict UAL748 from climbing. However, he is not presently 'released for climb'; he cannot be climbed until he reaches the BLOCK sector controller's airspace. So she presses the 'call' button for the tactical position at DORP sector:
DORP: Dorp sector?
BLOCK: Yeah, it's Block here, reference United 748?
D: Yeah?
B: Can I have him released for climb please?
D: Stand by.
...
D: Yep, United 748, released for climb.
B: Thanks. (click)
...then presses the foot switch:
United 748, climb flight level 150.
Climb flight level 150, United 748.
The DORP sector controller has had a scan around their airspace, seen that there is nothing above UAL748 to restrict further climb, so he gets another 5,000 feet, and a bit more speed and fuel efficiency out of his turbofans.
That's all very nice, but why the stop at FL150? Because the DORP sector controller can only authorise a climb to the top of their airspace, which in this case is FL150. What if UAL748, in a fairly unusual turn of events, is a Gulfstream IV (which climbs like a rocket)? 5,000ft is a nice bit of extra and early climb, but the performance of the aircraft means it may well be able to get up to FL220 before it even reaches BLOCK sector's airspace.
Maybe the BLOCK sector controller is in a really good mood today (or just has nothing better to do). Once she's spoken to DORP, instead of giving UAL748 the climb to FL150 she then taps the 'call' button for the tactical position at STACKER sector:
STACKER: Stacker sector?
BLOCK: Block sector here, reference United 748.
S: Go ahead.
B: He's 25 miles from the sector boundary, tracking west.
S: [Radar] contact.
B: Requesting coordinated climb, flight level 220?
S: Roger, flight level 220 is coordinated for United 748.
B: Thanks. (click)
Of course, the STACKER sector controller would have to be in a good mood too. BLOCK sector's controller has requested permission to climb UAL748 through STACKER sector's airspace on its way to BLOCK sector, and the controller at STACKER has given it. The climb has been coordinated. UAL748 gets to climb all the way up to its cruising level from departure, without having to level off on the way.
UAL748: Block sector, United 748, climbing flight level 100.
Block: United 748, Block sector, climb flight level 220.
UAL748: Climb flight level 220, United 748, thanks very much.
Gratitude added (grumble grumble). In reality the BLOCK controller's request to the STACKER controller would probably be a bit less formal: "can I climb him in your airspace please?"
Descents may also be coordinated (or released) like this, and a sector may similarly ask for an aircraft to be 'released for turns' if they want to vector it before it has reached their airspace.