Command supers (abbreviated as cmd super or cmd EX) are character-specific supers that require a sequence of button inputs. An example is Morrigan's Darkness Illusion: "LP LP 6 LK HP", this may look familiar as it's the same input for Akuma's Raging Demon in other various Capcom games.
These command supers have various unique properties. First, they can cancel any normal or command normal move on the ground (or in the air if applicable) at any point during the move, ignoring all cancel restrictions. This includes when an attack is chained into (explained later).
You must input the attack sequence for them as listed, however not all attacks in the sequence need to actually happen in order for the super to come out. Directionals are special in that diagonals count towards the sequence, for example Morrigan's DI can be input 1LP 1LP 3 3LK 3HP, the down+forward (3) diagonal will trigger the forward in the sequence.
Additionally, any directional input may be pressed or held at any time during the sequence, as long as the correct direction(s) is pressed when the sequence calls for it. For example you may hold backwards while inputting Morrigan's DI provided you tap or hold forwards after the second LP input, after that forward motion is input you may again press or hold any direction during the rest of the input. Diagonals are accepted for the directional input, but only as a singular input for the horizontal or vertical. An example would be "1LP 2LP 6 4LK 7HP" or "1LP 2LP 3LK 3HP", either will result in a DI. With the second example listed the diagonal 3 is counted as a forward input (which means 6 is accepted).
As stated, cmd supers can cancel any normal or cmd normal attack, even if it's chained. Normally in VSav you would not be able to special/super cancel a chained attack (from light to mid and/or heavy attack) example c.LK c.MP c.MK would not allow you to cancel c.MK. But with cmd supers ignoring normal cancel rules, it becomes possible. An easy example is with Lei-Lei's Tenraiha: "LK HK MP MP 8" where if you input the command quickly enough the LK will chain to HK (which is not normally special/super cancelable) and then the cmd super will cancel the HK, creating a natural combo.
Comboing non-cmd-supers is possible when cancelling normally cancelable attacks, which is common with links. There are some characters who can chain into an attack that gives significant enough frame advantage to then link a super off of the next hit.
Each time an attack is pressed for the cmd super an invisible timer starts to count down. The timer is only 9 frames (extremely fast, a jab typically animates for 9F), if the timer reaches zero then the input sequence fails and the cmd super will not execute. Directional inputs do not reset the timer, only attack inputs do. Using the example of DI, the LP LP LK and HP must all be pressed within 9 frames of each other with the forward not counting. That makes the maximum amount of time you can spend inputting a DI 27 frames (because HP ends the sequence and forward doesn't reset the timer). In other words there can not be a prolonged gap between the second LP and the LK. The minimum amount of time to input a DI would be six frames (because you must release LP to press it again).
Piano / Slide
Command super inputs do not have to be released per button press (unless a button occurs twice in the sequence). It is therefor possible to hold any and all parts of the sequence when inputting a cmd super. What this also allows is kara-type button inputs, such as sliding and pianoing. For example, Morrigan's DI can be input by tapping LP once, then LP~6~LK~HP, in other words it's like pressing only two buttons, where the first LP is the first button and the entire rest of the sequence is the "second button". On a stick, this could be done by tapping (and releasing) LP, then fat-fingering the LP and LK with the index finger, pressing them just slowly enough to squeeze a 6 in between, then hitting HP at almost the same time as the LK.
An example of skipping would be with Lei-Lei's Tenraiha: "2LK (2HK) 2MP 2MP 8" where if pressed quickly enough you will chain c.LK to c.MP (skipping the c.HK entirely) then cmd-super-cancel the chained c.MP. Another valuable example is with Gallon's Moment Slice "LP MP 6 LK MK" where if you input it quickly enough you can skip the second half of the sequence and create a natural combo c.LP c.MP Moment Slice, by inputting "2LP 2MP (3LK 3MK)" (because diagonal down+forward counts for the forward input).
There are other ways to "skip" or "hide" inputs by sliding them within other inputs, such as pressing two buttons at the same time. Morrigan's DI may be input by pressing "1LP, 1MP+LP, 3MK~LK, HP" which results in c.LP c.MP c.MK DI.
Or, you may do "1LP 1LP 3MK 3LK 3HP" which results in c.LP c.LP c.MK DI.
Lei-Lei could also do "1LP (1HK) 1MK (MP MP 8)" which results in c.LP c.MK Tenraiha.
Hiding (Empty) Inputs
There is also ways to "hide" inputs because of the 9-frame window of execution per attack input. For example, during blockstun you can input the sequence for the cmd-super, ending with the last input once you leave blockstun. Likewise you can whiff an attack and input the sequence during the whiff.
There's also a 3-frame window when jumping where you can not perform any action, this gives you enough time to input one button of the sequence without performing any action, which is useful for airblocking into DI/LI.
Some characters have an uncancellable window on their dashes where they can input parts or all of their cmd-super sequence. Common examples being Demitri's Midnight Pleasure during a dash, or Lilith inputting two LP's for LI during her dash startup.