The unlocks in this game are 100% random.
You unlock a new thing every 2nd level up, but the game does not tell you which one. You need to find out yourself. For headgear, tops, and pants, a green circle will appear on the category button to indicate a new unlock.

Things you can unlock are:

• - Headgear
  
• - Tops
  
• - Pants
  
• - Camos/colors
  
• - Patches for the shirt shoulder
  
• - Backgrounds for the armory (not shown in this guide)

This guide shows all the cosmetic items, unclouding the few that are unlocked by default. This is all the customization currently available in the game to the player.

Everything in the following categories is unlocked by default:

• - Face
  
• - Headset
  
• - Armor
  
• - Vest
  
• - Backpack

Items not shown in this guide:

• - Gloves (2 items)
  
• - Elbow pads (1 item)
  
• - Knee pads (1 item)

Tops, pants, vests, and backpacks are presented in a way that lets you see their slots and layout.

Higer resolution images are available here:
https://imgur.com/gallery/jBcrKrS