While a Choice Specs set allows Reshiram to destroy stall, this set allows for more flexibility and greater staying power in exchange for a drop in damage output. With its ability to switch attacks, Reshiram poses a more significant threat to slow, bulky teams, which rely on pivoting to deal with it. This set thrives on forced switches, and against a hyper offense team Reshiram's bulk often won't be enough; it's exceedingly susceptible to getting revenge killed unless it runs Tailwind or Flame Charge.
Due to Reshiram's impeccable two-move coverage, an all-out offensive set is redundant; unfortunately, before BW2 Reshiram lacked the movepool to run an effective 3 Attacks + Support set. BW2 gave Reshiram exactly what it wanted: Roost. With reliable recovery, Reshiram can capitalize on the switches that it causes to get rid of hazard and Life Orb damage.
This set's third moveslot, its support move, is one of the defining factors that determine how the set will play. Running Tailwind turns Reshiram into a momentum snatcher for its team, setting up on a forced switch and potentially sweeping. Although Modest Reshiram hits 558 Speed—which easily outspeeds most relevant threats—after a Tailwind, a Timid nature is still usable, as there will be times when Reshiram will not have time to use Tailwind. If maximum Speed is of no concern, one could run an alternative EV spread of 80 Atk / 252 SpA / 176 Spe, which allows Reshiram to outspeed the fastest commonly seen Ubers Choice Scarf user, Terrakion. However, 4 Def / 252 SpA / 252 Spe is generally better, because when Tailwind isn't in effect the loss of Speed is noticeable. Running Tailwind also allows Reshiram to support its team; powerful offensive wallbreakers, such as Rayquaza, mixed Zekrom, four attacks Mewtwo, and Lustrous Orb Palkia, become far more threatening with their Speed doubled. Alternatively, Roost can be run to increase Reshiram's longevity greatly; between hazards, Life Orb, and miscellaneous attacks, Reshiram's HP will drop quickly without it. Running Roost will transform Reshiram from a sweeper to a tank, taking hits, healing up, and returning fire.
As for the last slot, the main choice is between Outrage and Stone Edge. While Stone Edge allows Reshiram to annihilate Ho-Oh, Outrage is generally the better option due to its increased power against Latias, Chansey, and others. It can also serve as a backup sweeping tool if one is running Draco Meteor, as a Reshiram at -2 SpA will often want a solid STAB option to continue a sweep if need be. If one is running Tailwind in the third slot, Roost becomes a viable option in the last slot as well. Running the combination of Roost and Tailwind will turn Reshiram into a team supporter capable of facilitating sweeps repeatedly. However, the lack in coverage is noticeable and generally Outrage or Stone Edge will be of more use.
As will always be the case with Reshiram, sun support is mandatory for this set. Without it, Blue Flare will often be weakened by rain. Despite having Roost, Reshiram still appreciates Rapid Spin support, as the more turns spent recovering, the less turns are spent attacking. Entry hazards on the opponent's field are, as always, important. They allow Reshiram to get many of the KOs that it would otherwise miss. Paralysis is also incredibly important, especially if one isn't running Tailwind. Due to Reshiram's moderately-low 90 base Speed, it needs the support to be able to perform to the best of its ability; Groudon, Dialga, Arceus-Grass, and Ferrothorn can all spread this helpful status reliably. Finally, this set also appreciates not only the aforementioned wallbreakers, but a fast cleaner that can take advantage of the holes that Reshiram makes in teams.