Vertical

c only ever climbs and d only ever descends, postfix included. A c target below the aircraft is refused with a note on frequency rather than flown as a descent, which covers c100- from FL200. A target level with the aircraft is accepted and means "maintain".

The number-pad form / is the exception, and deliberately so. One character cannot say climb or descend, so /090 takes the aircraft to FL090 from wherever it is and the server has nothing to check: the level that c 090 refuses at an aircraft above FL090 is one that /090 simply flies. Type the letter form where you want that check.

A level above the type's service ceiling is lowered to the ceiling rather than refused, with a note saying so. The rest of the line is flown either way: a dropped or lowered altitude does not take your turn or your speed with it.

A level of zero or below is different, because nobody clears an aircraft to the ground. It usually means digits landed where they were not meant to: 2000 on its own is the number pad's descent to FL000, when a squawk was probably intended. The server refuses the whole line, says what the number was read as, and applies nothing from it.

A descent to an altitude you clear an aircraft to never asks it for more than idle thrust can give while it holds the speed you gave it. The final approach is its own case, flown on the glidepath whatever that costs. On most types the tabled descent rate is already within that and nothing has changed. On some the table assumed a steeper path than the aeroplane can hold its speed on with the levers closed, so they descend at their own idle glide instead, which is shallower. They take a little longer to get down, and one you have cleared down fast and high reaches the altitude later rather than arriving there on speed for free.

The Airbus A320 descends close to Airbus's published idle rates. At 250 knots below FL100 it comes down at about 1600 feet per minute, and at 300 knots around FL150 at about 2550. The Boeing 737-800 descends close to Boeing's published idle rates, and the 737-900 uses the same engine figures. With the speedbrake out the A320 and the 737s come down steeper than that. See What an arrival does by itself.

The + / - postfix belongs to c, d, s and the number pad's /. It does not attach to abv, blw, smin or smax, which already carry the direction in the verb: abv 100- is refused rather than quietly read as a floor. It goes with the short spelling and not the full one, so c100+ is a floor and c 100+ is not a command.

On the slash it is the easiest of the four to read, because / names no direction for a limit to contradict: /100+ is a floor and /100- a ceiling, whichever side of FL100 the aircraft is on.