Items in Diablo Immortal are separated in two different Equipment Slots: secondary and primary. Secondary Equipment Slots consist of the Amulet, both Rings, Gloves, Belt and the Boots, whereas Primary Equipment Slots consist of Helmet, Chest, Shoulders, Pants, Main- and Off-Hand.
All items, with the exception of the Charm, can be upgraded and have their ranks increased via the Blacksmith; the rank grants additional attributes up to Rank 5 and afterwards additionally affixes are unlocked.
Secondary Equipment Slots can roll up to three Sockets and can only be socketed with Normal Gems, whereas Primary Equipment Slots can roll up to one Socket and can only be socketed with Legendary Gems. A fully decked out character can therefore equipp a total of 6 Legendary Gems and 18 Normal Gems.
Until now, with the maximum Item and Character Level Cap of 45, and with the limited time players had to test the Technical Alpha it is unclear, if, down the road, further differences arise between these two Equipment Slots. It's likely possible that with additional ranks different "affix families" emerge, which is something Reforge Stones are currently hinting at.