Event operations are part of various limited-time events. Based on their scale, event operations are divided into major and minor.
Major event operations are divided into three categories, main, extra, and secret. The operations are distinguished with an event-specific prefix (e.g. "GT" for Grani and the Knights' Treasure, "OF" for Heart of Surging Flame – known in-game as Obsidian Festival) and each event consists of an average of eight to ten main operations.
- Main operations are related to the event's plot and allows the player to farm the event's currency in addition to upgrade materials which has a higher drop rate than in Main Theme operations, especially T3 ones. However, main event operations do not have a Challenge Mode version, thus they are solely meant for farming the event currencies. As such, it is more than recommended to stock up on the materials these events provide while they are running.
- Extra operations, distinguished by "EX" appended between the prefix and number (e.g. "GT-EX-1" and "OF-EX3"), are unrelated to the event's plot, though some do have story content that are relevant to the plot. Extra operations are more difficult than main ones and have no rewards aside of the event's currency and the usual one Originite Prime as the first drop, thus they serves as an additional challenge for the player. Unlike main event operations, extra operations have a Challenge Mode version, which gives out additional Originite Prime when cleared for the first time.
- Secret operations, distinguished by "S" appended between the prefix and number (e.g. "DH-S-1"), are loosely related to the event's plot and are not immediately available; they must be unlocked by fulfilling certain conditions during the event. Secret operations are much more difficult than main and extra ones due to the addition of certain handicaps, but like the latter, have no rewards aside of the event's currency and the usual Originite Prime as the first drop. Secret operations have a Challenge Mode version like extra operations, which gives out additional Originite Prime when cleared for the first time.
- The Mini-Annihilation, distinguished by "MO-1" appended before the prefix (e.g. "DM-MO-1") is essentially an operation with much more enemies, around the same as Annihilation operations which extends to its rules (10 Life Points, no automatic DP generation, and the DP cap of 999). Like extra and secret operations, the Mini-Annihilation have no rewards aside of the event's currency and the usual Originite Prime as the first drop.
After the major event ends, their respective operations will be added into the Terminal at various points depending on their designation, allowing them to be replayed anytime:
- Intermezzi events' operations are added as soon as it ends.
- Side Story events' operations will be added after their respective rerun ends.
However, the event operations in the Terminal are subject to the following changes:
- Operations with a particular upgrade material as the regular drop have a noticeably lower drop rate for the material than when it was featured in the event (around the same rate as Main Theme operations), and therefore no longer extremely advantageous. To offset this, some of them now also have various special and/or extra drops.
- Event currencies and one-time rewards are not awarded from operations that award them when it was featured in the event.
- Because of this, main OF operations (OF-1 to OF-8) whose sole purpose is to farm the event currencies do not consume Sanity to run (albeit still costing one Drill Plan for a practice run) but have no rewards whatsoever. This makes OF operations the best place to boost the player's missions' progression.
- Progress towards medals that require clearing Operations can be obtained even after the event has ended. However, some medals are permanently unavailable after the event's run, generally those that specifically word "During the event..." in their unlock condition.
Vignette operations are unrelated to the event's plot and while they have upgrade materials as a regular drop, the drop rate is usually around the same as in Main Theme operations, which is why it is recommended to not put aside one's other farming plans for these materials as one would for a major event. However, Vignette operations tend to be significantly more difficult than major events' operations, somewhere around or even above the difficulty of major events' extra operations, and also have a Challenge Mode version. This means that Vignette operations should be only completed once, including their respective Challenge Modes, to claim its first clear rewards which includes the usual Originite Prime, the event's currency, and plaques.
Prior to Children of Ursus, Vignettes interspersed their operations through the Main Theme episodes instead of using their own node map in the same fashion as major events. Ever since then, however, Vignettes have been using their own node map.
While major events only lets one unlock the story interludes in sequence, following the story, Vignette interludes are unrelated to each other (with an exception to Ancient Forge ones), available from its own menu and can be unlocked and read from any order.