This is the ammo for the Planting Tool. It can be found in abandoned bases or made in a Seed Modification Machine.
Normal seed magazines (with no enhancements applied) are made by placing the raw crop in one of the slots and pressing 'Extract'. To enhance a seed type, place two stacks of the same crop type and enhancement into both slots and press 'Extract'. The amount of seeds extracted can be upgraded via the Botany Tech Tree.
The Magazines are single use, meaning once it runs out of seeds, it disappears. Multiple can be equipped and can be toggled between using the scroll wheel.
Extraction can be done with all food bearing crops of all tiers: