REQ Packs, AKA Cock Blocks, are the only way to get cosmetics (even the promotional items are contained in Cock Blocks!) in Halo 5, as well as a way to get weapons, vehicles, and power ups for use in Warzone.
Cock Blocks can be found in these styles.
Gold Cock Blocks cost 10000 Block Points, and contain items that range from Decently Findable to, apparently, Fucking Unobtainable. Gold Cock Blocks will contain 2 guaranteed cosmetics, special loadout weapons, armor mods, or certs.
Silver Cock Blocks cost 5000 Block Points, and contain items that range from Overly Abundant to Highly Sought After. Silver Cock Blocks will contain 2 guaranteed permanent items.
Bronze Cock Blocks cost 1250 Block Points. They're shitty and contain only Overly Abundant items, with no guaranteed permanent item.
Level Up Cock Blocks are earned by increasing your SR. They're like Bronze Cock Blocks, but can contain Decently Findable and Highly Sought After items. These don't have permanent items.
Commendation Cock Blocks are earned by completing commendations. The named ones contain a Highly Sought After emblem. If it's from a Mastery Commendation, the emblem will instead be Extremely Scarce. If it's the Arena Mastery or Weapon Mastery (both Mastery Mastery commendations), the emblem is rumored to be Fucking Unobtainable.
Premium Cock Blocks are a way for 343 Industries to take your money. If you bought the Halo 5: Gradius Limited Edition, you will get these for free. These contain items that range from Decently Findable to Fucking Unobtainable, and there is a decent chance of actually getting an Extremely Scarce or Fucking Unobtainable item. These also have THREE guaranteed permanent items.
Promotional Cock Blocks are obtained by entering special codes. These contain a special item, which usually is of Extremely Scarce rarity, though the Cock Block from the beta has the pure exclusive emblems marked as Fucking Unobtainable. The "pretentious bastard" Cock Block contains the FOTUS armor and helmet, as well as the pure exclusive Vanguard emblem. All of these are Extremely Scarce.