There are 8 labels used on this site related to obtainability of cards:
Unobtainable cards are not yet confirmed to have been acquired by any player through legit play, though certain enemy characters (such as in campaign or dungeons) might have them.
Every known card is considered unobtainable until a reproduceable method to acquire it has been found. This method is allowed to rely on random chance.
There have been occurrences where as-of-yet otherwise unobtainable cards have been given to players by an administrator, being given a card by an administrator is not considered reproduceable. (This has happened once, the card given was Potent Healing Potion which was not at the time known to be obtainable)
Obtainable cards can be acquired in-game through legit play. If there is no prefix in front of Obtainable in the acquisition section of a card, then the card is assumed to be Standard Obtainable.
Example: Redling is considered obtainable since it can be rewarded from bronze card packs and as a mission reward from certain campaign missions as well as from certain chests.
Standard obtainable is the most common type of obtainability for cards. Standard obtainable means the card can be acquired from packs or be evolved to from cards that can be acquired from packs, they can often be rewarded from missions, and can often be commonly found in certain chests.
Example: Frozen Edge Master has standard obtainability since it and its prevolutions can be acquired from card packs. it can also be acquired from certain chests.
A difficult obtainable card is any card which is obtainable but cannot be acquired from packs or be evolved from any card which can be acquired from packs.
A card with limited obtainability can only be acquired in a limited amount through legit play.
if this amount is 1 it is also called unique.
Example: Holy Sacrifice is considered unique and limited obtainable.
A selectively obtainable card is obtainable if a correct choice is made, generally any choice relating to selectively obtainable cards will result in acquiring 1 such card and losing the chance for other selectively obtainable cards.
A selectively obtainable card also has difficult obtainability in the sense that packs do not drop them, though selectively obtainable cards do not generally require much effort to get, unlike most other difficult obtainable cards.
Also note that all known selectively obtainable cards can be bought from the VIP shop for a rune cost, therefore all selectively obtainable cards are also VIP obtainable for anyone.
A randomly obtainable card can only be acquired by some random players, it is generally impossible to know beforehand whether you are one of the players which can get it, randomly obtainable cards are generally also limited obtainable and unique.
Example: Swamp Dragon Egg can only be acquired as a reward from a campaign sidequest. the sidequest cannot give rewards multiple times and it can also reward otherwise obtainable cards. Therefore the players whom get Swamp Dragon Egg are the only players for whom it is obtainable.
Some cards are available only once you paid to become a VIP. There is no other way to acquire those cards today than to buy them for runes from the Revolving Card Shop, which is unlocked when you become a VIP.