Downloadable content, usually abbreviated as DLC, is additional content available to download after the initial release of a game. DLC was first confirmed for Super Smash Bros. 4 in the "Super Smash Bros. for Wii U: 50-Fact Extravaganza" video presentation on October 23rd, 2014. Downloadable content for the game includes playable fighters, stages, and even additional features. Paid content in Super Smash Bros. for Wii U is found for purchase in the Nintendo eShop, and in Super Smash Bros. for Nintendo 3DS is found within a game-based DLC shop.
In the Super Smash Bros. Ultimate Nintendo Direct on November 1, 2018, series director Masahiro Sakurai confirmed that paid DLC would be available for Ultimate. It will come in 5 bundles each containing one fighter, one stage, and several music tracks. These bundles, which will also be available through the Fighter Pass, will be released periodically between the game's December 2018 release and February 2020. Additionally, Piranha Plant is available for free as a limited time early purchase bonus, and will be subsequently available for purchase. On November 6th, Sakurai announced on his Twitter account that the DLC line-up was now complete, and it was decided by Nintendo. . However, it's implied that Sakurai will still have the final choice on whom gets added.
Super Smash Bros. 4
Main article: List of updates (SSB4-3DS)
Main article: List of updates (SSB4-Wii U)
Updates to the game are downloadable at no extra cost. The game must be updated to the latest version to purchase DLC, which is done automatically after starting the game from the console's main menu. Most updates also adjust game balance and mechanics, while fixing glitches and other errors as well. Change-logs for all updates are available at the links above. Nintendo has not released any detailed information as official announcements.
As with any other game, DLC data gets added via free updates. Completing a transaction to receive DLC is merely a "ticket" to grant access to said content. What this means is that DLC not purchased in an updated version is still technically installed but locked, and thus is accessible indirectly. Content that is locked can be seen and played against with opponents connecting via a system that has DLC paid for (in "With Anyone," "With Friends," and local Nintendo 3DS Versus modes). Without another person with DLC, locked content can still be seen via using amiibo as Figure Players, and saving uploaded or shared replays.
Similar to clone characters, DLC characters are recognized by being placed in a particular area on the character selection screen, regardless of which franchise from which they hail. When more than four DLC characters are on the 3DS version, and all other characters are unlocked, the DLC characters are labeled "Extra Fighters" on a separate page.
DLC characters also lack custom moves, and all have a generic, previously unused Palutena's Guidance scene. While there are no official reasons for this, the former is presumably due to time and budget constraints, and the latter is presumably due to repeatedly bringing back voice actors each time a character is released.
Mii Fighter costumes
Additional costumes for the Mii Fighters are available as paid DLC.
Trophies of fighters for All-Star Mode and Classic Mode are included upon purchasing the fighter, as well as additional trophies:
Super Smash Bros. Ultimate
There will be five DLC packs released, each containing 1 unique (non-Echo) fighter, 1 stage, and its corresponding music tracks. Each pack will cost $5.99 each. There is also a Fighters Pass, available for $24.99, that will automatically obtain DLC as soon as it is released. Additionally, purchasing it will award a Mii Costume for Swordfighters based off of Rex from Xenoblade Chronicles 2. Separately, pre-purchasing the game off the eShop or registering a physical copy of the game with My Nintendo before January 31, 2019 will award Piranha Plant for free when it is released. It will be available as paid DLC as well after it is released.
Downloadable characters were chosen by Nintendo, although the final decision on whom to add was made by Sakurai. 
Note that DLC bundles are incrementally discounted based on the amount of content within them.
Downloadable characters can be bought for one version at full price or both versions discounted. There are three levels of pricing for characters: veteran characters are the least expensive, the sole Nintendo newcomer is slightly more expensive, and third-party characters are the most expensive (and are each packaged with a stage). Additionally, characters can be bought in bundles, which includes any stage a character was included with. Characters are priced at:
Stages can be bought for one version at full price or for both versions discounted. Stages that come bundled with a character cannot be purchased separately. Stages are priced at:
Mii Fighter costumes
As with characters, downloadable Mii Fighter costumes can be bought for one version at full price or for both versions discounted. These can also be bought individually, or in a bundle that includes all Mii costumes added in a particular update. The first 3 bundles all cost the same. They are priced at:
Full DLC Collections
Also available for purchase are collections that include all DLC released in a particular update. These vary in price depending on the amount of content.
DLC Trailer Video
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