Each container has four sets of Materials.
You can choose 3 of them for free.

Those sets contains the following:

• 1000 units of Common Materials: Medication, Alloys, Spare Parts, Fuel.
  
• 375 units of Uncommon Materials: Composite Materials, Chemical Components, Encryption Codes.
  
• 290 units of Rare Materials: Microchips, Blueprints OR 230 units of a Very Rare Material, Secret Prototypes.

Materials are required for some operator upgrades and for skill research.