Different Packs have different probabilities associated with acquiring Items. The Items that show up most to least often are as follows: Common, Rare, Epic, Legendary.
An Items Rarity does not have any effect on gameplay, and is merely used to indicate how often that Item, with it's specific Athlete and Multiplier combination, shows up in the Item Player Pool Distribution. All Item and Pack probabilities are determined by GameBlazers proprietary Item Distribution Framework.
Please refer to the Packs page in-app for more details on specific rates and probabilities associated with each Pack.