In Cardfight, one of the main obstacles to winning a game is building and controlling the rearguard formation. Clans which make use of call skills have to play carefully to ensure that the card they called does not interfere with their current strategy, and if a poor line like two units whose sum power is 14000 versus an 10000-power opposing vanguard is set up, then fighters will usually have to voluntarily retire one or more parts of the formation to fix the line. This hurts card advantage, and to avoid it often requires careful planning from the very beginning of the match.
Broadly speaking, the concept of an open or closed game is a measure of how easily the rearguard formation can be modified or replaced, and it plays an important role in controlling the usefulness of a card. Gigantech Charger is a unit that is usually called to the rearguard--because of this, to use his skill to its full potential Charger has to be called with one open space in the backline and two in the front (one for him and one for any potential grade 0, 1, 2 or 3 he calls.) Because of this, Gigantech's presence in the hand effectively locks down three spaces from being used or otherwise encourages early intercepting, limiting how much offensive power a deck that uses him can claim at the early stages of the game, while handing control of a match's speed to the opponent. These factors are all rooted in the Royal Paladins playing a closed game; only a limited pool of units can be taken off the field once they touch it. So despite having a wider range of superior call skills than most clans, changing their formation enough for the increase in advantage to become meaningful is difficult.
Contrast Great Nature or Bermuda Triangle. Great Nature cards can be retired freely, with the Hamsuke series and Lox line adding replacement cards to the hand. While Leo-pald's limit break locks down the field somewhat, it also works with cards like Loop-the-Loop, Duckbill to increase advantage and make the field more open by taking the strain away from calling. Bermuda Triangle meanwhile, bounces rearguards back to the hand and has skills to call different rearguards mid-battle, rarely if ever playing with a fully-locked field.
Whether a clan is playing an open or closed game is critically important. Up until Spectral Duke Dragon's limit break opened it for them, the Gold Paladins played with an entirely closed field, due to their superior call skills requiring open rearguard circles. Cards put onto the field are still lost forever with Duke, but now the spaces that they occupy can be freed up once more. This is why Blackmane Witch is such a critical promo for the clan, because her skill allows the field to be rearranged freely in addition to giving Duke no bad opening ride. Similarly, Badhabh Catha is much more useful to the Shadow Paladins than Gigantech was to RoPala because it's possible to do a near-full field wipe at almost any time in the main phase, so those three spaces that Gigantech controlled for the Royals aren't an issue to the Shadows. Nemain would be a poor card to Duke Dragon, but is one of the best support cards Phantom Blaster has because she is geared for a clan that plays with a field that is reopened during the main phase rather than battle phase. By contrast, Viviane and Lop Ear Shooter would never function as well in ShadowPala as they do in GoPala, because they are designed for fields that open in the battle phase, and create repeating attacks or superior rides rather than fuel for another skill. If Bermuda Triangle were to receive its own Nemain, then the popularity of the deck would skyrocket because of the appeal of being able to recycle a superpowered Aermo-Dark Cat hybrid with Weddell, Perla, Flores or Raindear.
Of course, even among a given clan, how open or closed the game is can vary wildly between decks. Among Oracle Think Tank, the game is only open if the Goddess of Flower Divination, Sakuya is included to return called units to the hand. While Tsukuyomi's draw skill can certainly make up for the cards lost by retiring rearguards normally, it's more optimal for her to stack her skill on top of a well-prepared field. Amaterasu meanwhile, cannot afford to make mistakes regarding formation because she has difficulty replacing units without suffering a loss in advantage. This makes Dark Cat a critically important card for the latter two, because his draw skill makes him a free 7000-power booster and one card that will never have to be replaced from the back line. But to Sakuya, the Cat is not as useful because she plays with an open field in the first place, so reconfiguring boosting units is much easier to manage than in the other decks.
Among the Gold Paladins, Blond Ezel decks have only Garmore and Blackmane Witch to open their field, and even then Blackmane chiefly serves to compensate for the poor calls made by Viviane and similar units. Naturally, Spectral Duke Dragon decks have the titular Duke Dragon's field wipe to rely on as well as the Vortimer series, but because this field wipe can only happen in the battle phase and the previous cards have to be played in a specific sequence without missing beats, the skills have less utility than Blaster Dragon's. Royal Paladin has similar moves to change the formation around, with Pongal being freely replaceable for a counterblast 1, Alfred's counterblast 3 being able to call rearguards with no loss, and before his restriction Barcgal's superior ride sequence was also a part of this opening of the game. These limited reopenings are part of the clans' inherent advantages, with the limitations also being their weakness when compared to the aforementioned Bermuda Triangle and Shadow Paladin clans. Moreover, Pale Moon has what is likely the most open game in Cardfight, as their skills are universally oriented toward swapping units in and out of the soul, restructuring their formation at-will and even turning weaker rearguards into all-out attacks with soul-based power increases. Units like Jumping Glenn can be combined with Quantum Magician or Nightmare Doll, Alice to make them fit into either attacking or boosting roles based on the situation, that would otherwise be limited to units of particular grade. So Pale Moon's level of openness comes not just from being able to change the formation, but also from being able to squeeze otherwise-inappropriate cards into differing roles.
How open or closed a game is needs to be considered carefully when selecting or changing clans, and in approaching new deckbuilds of a given clan. A cardfighter that has just gone from Spectral Duke to Ezel may be surprised by how carefully they have to play, and a ThinkTank fighter changing from Tsukuyomi to Sakuya-Amaterasu will have to play in a fundamentally different manner to get the results that they are used to. In closing, make sure to account for this factor when developing your strategy, as no one wants to be caught off-guard by their own lack of or excess freedom.